Add tox test config.

[asford]

Add a basic tox configuration to execute tests against
multiple python versions in a local development environment.

Tox is used to initialize an isolated python environment for tests,
including any required external dependencies.

This config uses tox-conda to support multiple python interpreter
versions.

---

Possible options:

• Add tox notes to README.md contributor instructions.

• Convert actions workflows to use tox for different python versions.

• Add basic tox config for different python versions

• Remove py311 from tox env list, currently broken

[jaltmayerpizzorno]

@asford, does py38 for you? For me, py38 fails (but py311 succeeds).

py38 develop-inst-noop: /home/juan/project/slipcover
py38 installed: absl-py==0.11.0,astunparse==1.6.3,attrs==22.1.0,auditwheel==4.0.0,cachetools==4.1.1,click==8.1.2,cloudpickle==1.6.0,colorama==0.4.3,commonmark==0.9.1,coverage==6.3.2,cupy-cuda111==9.5.0,Cython==0.29.28,execnet==1.9.0,fastrlock==0.6,find-libpython==0.2.0,Flask @ file:///home/juan/tmp/flask,flatbuffers==1.12,fonttools==4.29.0,gast==0.3.3,google-auth==1.18.0,google-auth-oauthlib==0.4.1,google-pasta==0.2.0,GPUtil==1.4.0,grpcio==1.32.0,gviz-api==1.9.0,h5py==2.10.0,hypothesis==6.39.3,importlib-metadata==4.11.3,iniconfig==1.1.1,joblib==1.1.0,Keras==2.4.3,Keras-Preprocessing==1.1.2,kito==1.0.4,matplotlib==3.5.1,memory-profiler==0.57.0,nulltracer @ file:///home/juan/project/nulltracer,numpy==1.19.5,nvidia-ml-py==11.450.51,opt-einsum==3.3.0,packaging==22.0,pluggy==1.0.0,protobuf==3.12.2,py==1.11.0,py-cpuinfo==8.0.0,py-spy==0.3.12,pyasn1==0.4.8,pyasn1-modules==0.2.8,pyelftools==0.27,pyerfa==2.0.0.1,Pygments==2.7.1,pynvml==11.4.1,pyperf==2.3.0,pysqlite3-binary==0.4.3,pystack-debugger==0.9.0,pytest==7.0.1,pytest-arraydiff==0.5.0,pytest-astropy==0.9.0,pytest-astropy-header==0.1.2,pytest-cov==3.0.0,pytest-doctestplus==0.12.0,pytest-filter-subpackage==0.1.1,pytest-forked==1.4.0,pytest-mock==3.7.0,pytest-openfiles==0.5.0,pytest-remotedata==0.3.3,pytest-xdist==2.5.0,requests-oauthlib==1.3.0,rich==9.13.0,rsa==4.6,-e git+ssh://git@github.com/plasma-umass/scalene.git@2026a24f7f16ac20da9d1ad306f6443db06a35b4#egg=scalene,scikit-learn @ file:///home/juan/tmp/scikit-learn,scipy==1.4.1,shyaml==0.6.2,six==1.15.0,-e git+ssh://git@github.com/jaltmayerpizzorno/slipcover.git@45dda2aceedc97e9902f48998276ed352e703da2#egg=slipcover,sortedcontainers==2.4.0,tabulate==0.8.9,tb-nightly==2.4.0a20200916,tensorboard==2.4.0,tensorboard-plugin-profile==2.3.0,tensorboard-plugin-wit==1.7.0,tensorflow-estimator==2.4.0,termcolor==1.1.0,tf-estimator-nightly==2.4.0.dev2020091601,tflite-runtime==2.5.0,threadpoolctl==3.1.0,tomli==2.0.1,torch==1.10.0,torchsummary==1.5.1,typing-extensions==3.10.0.2,Werkzeug==2.1.1,wrapt==1.12.1
py38 run-test-pre: PYTHONHASHSEED='249300130'
py38 run-test: commands[0] | pytest
WARNING: test command found but not installed in testenv
  cmd: /home/juan/anaconda3/bin/pytest
  env: /home/juan/project/slipcover/.tox/py38
Maybe you forgot to specify a dependency? See also the allowlist_externals envconfig setting.

DEPRECATION WARNING: this will be an error in tox 4 and above!
==================================================================== test session starts ====================================================================
platform linux -- Python 3.9.13, pytest-7.1.2, pluggy-1.0.0
cachedir: .tox/py38/.pytest_cache
rootdir: /home/juan/project/slipcover
plugins: anyio-3.5.0
collected 0 items / 3 errors                                                                                                                                

========================================================================== ERRORS ===========================================================================
___________________________________________________________ ERROR collecting tests/branch_test.py ___________________________________________________________
ImportError while importing test module '/home/juan/project/slipcover/tests/branch_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../anaconda3/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/branch_test.py:3: in <module>
    import slipcover.branch as br
E   ModuleNotFoundError: No module named 'slipcover'
__________________________________________________________ ERROR collecting tests/bytecode_test.py __________________________________________________________
ImportError while importing test module '/home/juan/project/slipcover/tests/bytecode_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../anaconda3/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/bytecode_test.py:2: in <module>
    from slipcover import bytecode as bc
E   ModuleNotFoundError: No module named 'slipcover'
_________________________________________________________ ERROR collecting tests/slipcover_test.py __________________________________________________________
ImportError while importing test module '/home/juan/project/slipcover/tests/slipcover_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../anaconda3/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/slipcover_test.py:2: in <module>
    from slipcover import slipcover as sc
E   ModuleNotFoundError: No module named 'slipcover'
================================================================== short test summary info ==================================================================
ERROR tests/branch_test.py
ERROR tests/bytecode_test.py
ERROR tests/slipcover_test.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 3 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
===================================================================== 3 errors in 0.09s =====================================================================
ERROR: InvocationError for command /home/juan/anaconda3/bin/pytest (exited with code 2)