Update pytest invocation and dependency version

This update makes sure that `kevlar/tests/__init__.py` is not included when invoking the test suite, as this causes problems (see pytest-dev/pytest#4404).
With this update and commit dab6418, kevlar is now compatible with the latest version of pytest (4.0.0), and so the dependency version info has been updated in the `make devenv` target.

Closes #284.

Makefile

@@ -9,7 +9,7 @@ help: Makefile
 
 ## devenv:   install software development pre-requisites
 devenv:
-	pip install --upgrade pip setuptools pytest==3.6.4 pytest-cov pytest-xdist pycodestyle cython sphinx sphinx-argparse
+	pip install --upgrade pip setuptools pytest>=4.0.0 pytest-cov pytest-xdist pycodestyle cython sphinx sphinx-argparse
 
 ## style:    check Python code style against PEP8
 style:
@@ -21,19 +21,19 @@ ext: kevlar/alignment.c src/align.c inc/align.h
 
 ## test:     execute the automated test suite
 test: ext
-	py.test --cov=kevlar kevlar/tests/*.py -m 'not long and not toolong'
+	pytest --cov=kevlar kevlar/tests/test_*.py -m 'not long and not toolong'
 
 ## test4:    execute the automated test suite with 4 parallel threads
 test4: ext
-	py.test -n=4 --cov=kevlar kevlar/tests/*.py -m 'not long and not toolong'
+	pytest -n=4 --cov=kevlar kevlar/tests/test_*.py -m 'not long and not toolong'
 
 ## testmore: execute the automated test suite, including longer-running tests
 testmore: ext
-	py.test -v --cov=kevlar kevlar/tests/*.py -m 'not toolong'
+	pytest -v --cov=kevlar kevlar/tests/test_*.py -m 'not toolong'
 
 ## testall:  execute the automated test suite, including all tests
 testall: ext
-	py.test -v --cov=kevlar kevlar/tests/*.py
+	pytest -v --cov=kevlar kevlar/tests/test_*.py
 
 ## doc:      build the documentation locally
 doc: ext