path type supports pathlib

[davidism]

• click.Path(path_type=pathlib.Path) returns the value as a pathlib object. Technically any class is allowed, but this is only intended for pathlib.
• Use os.fsdecode and fsencode to convert str and bytes. Since Python 2 isn't supported more, the value from the command line will never be bytes.
• Remove the internal path_type attribute, which was confusingly named (the path_type parameter is stored as type), and was just name.title().

• fixes click.Path returns str, not pathlib.Path #405
• closes path converter #1148
• closes Allow path_type of the Path type to accept a callable (#405_) #1234

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.