Allow default as keyword arg to os.environ.{get,pop}

[RasmusNygren]

Fixes #15193

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pyodide (https://github.com/pyodide/pyodide)
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: error: No overload variant of "get" of "Mapping" matches argument types "str", "str"  [call-overload]
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: note: Possible overload variants:
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def get(self, str, /) -> str | None
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def get(self, str, str, /) -> str
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def [_T] get(self, str, _T, /) -> str | _T