The pip-specific Path implementation has been removed, and all its
usages replaced by pathlib.Path. The tmpdir and tmpdir_factory fixtures
are also removed, and all usages are replaced by tmp_path and
tmp_path_factory, which use pathlib.Path.
The pip() function now also accepts pathlib.Path so we don't need to put
str() everywhere. Path arguments are coerced with os.fspath() into str.
* Add option to silence warnings related to deprecation of Python versions
* Move skip_if_python2 and skip_if_not_python2 decorator declaratios to test/lib/__init__.py and use them in test_warning.py
* Add tests to ensure that python version deprecation warning is shown correctly and can be silenced by a flag.
* Add new test to ensure that --no-python-version-warning flag does nothing if python version is not 2
