Speedup startup of Python by importing less

``_distutils_hack`` is imported by a ``.pth`` file at every start of a
Python interpreter. The import of costly modules like ``re`` and
``contextlib`` almost doubles the initial startup time of an
interpreter.

- replace ``contextlib`` with simple context manager and try/except
- replace ``re`` with simple string match
- move import of ``importlib`` into function body
- remove ``warnings.filterwarnings()``, which imports ``re``, too.

Fixes: #3006
Signed-off-by: Christian Heimes <christian@python.org>

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