Bump pytest dev dependency to 6.2.5

Resolves assertion rewriting issue when running in Python 3.10.
See pytest-dev/pytest#8540

setup.py

@@ -25,7 +25,7 @@
       ],
       extras_require={
           "test": [
-              'pytest==6.2.1',
+              'pytest==6.2.5',
               'pylint==2.6.0',
               'pytest-cov==2.10.1',
           ]