Meetup-client

- The `scan` function now also works for single page requests.
- Fixed the progress bar, such that also the first iteration is displayed.
- Updated README and example.

- The client now uses the 'meetup' namespace.
- The `scan` function now returns a generator object.
- Renamed 'MeetupClient' to 'Client'.
- Bumped minimum required Python version to 3.7.
- Updated dependency versions.

- The client now accepts a string or callable as access token.

- Initialize meetup-client.