Inline type stubs from typeshed

[MarcoGorelli]

closes #166

Stubs are taken from: https://github.com/python/typeshed/tree/main/stubs/olefile/olefile

Other changes:

• I used sys.version_info to do Python2 vs Python3 branching, then MyPy doesn't require extra type: ignores (it's also probably more robust?)
• I added a CI workflow which runs stubtest and mypy to check that the stubs remain valid, and that no extra nor extraneous symbols appear

Advantages of having the type stubs inlined are:

• when people pip install olefile, they get the stubs automatically
• it becomes easier to keep the stubs up-to-date

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• Similar PR in ultrajson: Inline stubs ultrajson/ultrajson#674
• This is part of a broader effort to improve typing across the Python ecosystem https://discuss.python.org/t/call-for-suggestions-nominate-python-packages-for-typing-improvements/80186