Pymongo-smart-auth

* Added support for PyMongo >= 4.0
* Renamed the `user` parameter to `username` to be consistent with the PyMongo 4.0 interface
* Dropped support for Python < 3.6

* Added support for Python 3.10
* Renamed the import to `MongoClient` to make the package truly drop-in, meaning you can just change your import to `from pymongo_smart_auth import MongoClient` when transitioning from PyMongo and _voilà_ (to avoid breaking compatibility with older code, `MongoConnection` can still be imported)

* Fixed client not properly initialising when authentication is disabled.

* Added support for `MONGO_CREDENTIAL_FILE` environment variable that should contain a string with a path to a credential file to use. The string may be a format string, in which case it will be formatted using the `kwargs` passed to the constructor.

* Added support for a `MONGO_AUTHENTICATED_URI` environment variable that should contain a fully authenticated MongoDB URI
* Added support for single-line credential files, which are assumed to contain a fully authenticated URI
* Tweaked credential file parser to ignore blank lines

* Updated `__init__.py` to avoid crashing with Python 3