Datetime-glob

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* Made code comply to latest mypy 0.790 and pylint 2.6.0
* Made docstrings comply with pydocstyle
* Explicitly target Posix (and develop on a Windows machine so that bugs related
to *e.g.*, path separators become obvious)
* Introduce continuous integration based on GitHub workflows
* Update package information (author email and URL)

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* Moved to github.com
* Moved datetime_glob.py to a module directory
* Added py.typed to comply with mypy

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* Added walk

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* Added as_maybe_datetime and as_maybe_date

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* Added fixed text to pattern segment

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* Added empty match as default value to match_segment