`dav_provider.py` docstrings refer to "UTF-8 encoded byte strings" where they should just say "strings"

[John T. Wodder II]

The docstrings in the file wsgidav/dav_provider.py make numerous references to "UTF-8 encoded byte strings" or similar — which can only refer to bytes values — but it seems that they should actually be referring to str values. In particular, some of the methods with such docstrings can clearly (based on their implementations) only operate on strs, not bytes!

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