Fix for Python 4: replace unsafe PY3 with PY2

[hugovk]

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but whichever it is, it will probably happen in 2020 when Python 3.9 reaches beta and work begins on Python 3.9+1.

This will be false for Python 4:

        """
        Test that supports breakpoint global marks on Python 3.7+ and not on
        CPython 3.5, 2.7
        """
        if sys.version_info.major == 3 and sys.version_info.minor >= 7:
            assert SUPPORTS_BREAKPOINT_BUILTIN is True

---

Found using https://github.com/asottile/flake8-2020:

$ pip install -U flake8-2020
...
$ flake8 --select YTT
./testing/test_pdb.py:856:44: YTT204 `sys.version_info.minor` compared to integer (python4), compare `sys.version_info` to tuple

---

• Target the master branch for bug fixes, documentation updates and trivial changes.
• [n/a] Target the features branch for new features, improvements, and removals/deprecations.
• [n/a] Include documentation when adding new features.
• [n/a] Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• [n/a] Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[nicoddemus]

Thanks! 😁