Support Python 3.8

[bittremieux]

Numba 0.46 doesn't support Python 3.8. See for example numba/numba#4712.

Example of failed unit test:

E       numba.errors.LoweringError: Failed in nopython mode pipeline (step: nopython mode backend)
E       Failed in nopython mode pipeline (step: analyzing bytecode)
E       list index out of range
E       
E       File "../../../miniconda/envs/test/lib/python3.8/site-packages/spectrum_utils-0.3.2-py3.8.egg/spectrum_utils/spectrum.py", line 616:
E       <source missing, REPL/exec in use?>
E       
E       [1] During: lowering "$76.14 = call $76.2($76.13, func=$76.2, args=[Var($76.13, /home/travis/miniconda/envs/test/lib/python3.8/site-packages/spectrum_utils-0.3.2-py3.8.egg/spectrum_utils/spectrum.py (616))], kws=(), vararg=None)" at /home/travis/miniconda/envs/test/lib/python3.8/site-packages/spectrum_utils-0.3.2-py3.8.egg/spectrum_utils/spectrum.py (616)
E       
E       -------------------------------------------------------------------------------
E       This should not have happened, a problem has occurred in Numba's internals.
E       You are currently using Numba version 0.46.0.
E       
E       Please report the error message and traceback, along with a minimal reproducer
E       at: https://github.com/numba/numba/issues/new
E       
E       If more help is needed please feel free to speak to the Numba core developers
E       directly at: https://gitter.im/numba/numba
E       
E       Thanks in advance for your help in improving Numba!
../../../miniconda/envs/test/lib/python3.8/site-packages/numba/six.py:669: LoweringError

As soon as a py3.8-version compatible Numba version (0.47) is available:

• Set the minimum Numba version in setup.py and the Bioconda recipe.
• Enable unit tests for Python 3.8 in .travis.yml.
• Include the Python 3.8 badge in the README and the documentation homepage.

[bittremieux]

Fixed in 91de987.