Add support for pathlib.Path

[jarekwg]

Django has made some moves in the past year to support pathlib.Path() with FileSystemStorage https://code.djangoproject.com/ticket/28428

For alignment, django-storages should head in the same direction.

Not proposing any breaking changes here (just yet), but would be nice if Path objects were acceptable parameters when feeding in keys and prefixes.

[tolomea]

For S3Boto3Storage we are currently wraping it with this

class BaseS3Storage(S3Boto3Storage):
    def _clean_name(self, name):
        name = str(name)  # convert any pathlib stuff to strings
        return super()._clean_name(name)

to deal with this

[jschneier]

This landed in #1200.