Pin pytest *before* 3.9.1 totally

Unfortunately they have released
3.9.2 before fixing issue 4181, which means we will have to
re-pin a third time after they eventually release with
a fix

Change-Id: I9b6fab1b535f883a035f002264c41ad0166790ab
Reference: pytest-dev/pytest#4181

setup.py

@@ -139,7 +139,7 @@ def run_setup(with_cext):
         package_dir={'': 'lib'},
         license="MIT License",
         cmdclass=cmdclass,
-        tests_require=['pytest>=2.5.2,!=3.9.1', 'mock', 'pytest-xdist'],
+        tests_require=['pytest>=2.5.2,<3.9.1', 'mock', 'pytest-xdist'],
         long_description=readme,
         python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",
         classifiers=[

tox.ini

@@ -15,7 +15,7 @@ install_command=python -m pip install {env:TOX_PIP_OPTS:} {opts} {packages}
 usedevelop=
      cov: True
 
-deps=pytest!=3.9.1
+deps=pytest<3.9.1
      pytest-xdist
      mock
      # needed only for correct profiling results