Set pytest-cov to >=2.6.1,<3 (#294)

setup.py

@@ -102,7 +102,7 @@ def get_git_commit_datetime() -> str:
     'mypy>=0.710,<0.8',
     'pydocstyle==3.0.0',
     'pylint==2.3.1',
-    'pytest-cov==2.6.1',
+    'pytest-cov>=2.6.1,<3',
     'pytest-sugar>=0.9.2,<0.10',
     'pytest-timeout>=1.3.3,<1.4',
     'pytest>=5.1.2,<6',