Reject invalid HTTP methods and resources

[andy-maier]

This change addresses issue #1018 (currently, any HTTP method is handled by returning success and metrics data, which causes network scanners to report issues).

Note, this needs careful review w.r.t.:

• Does Prometheus issue any HTTP requests that are now rejected.
• Does HEAD need to be supported.
• Does the /metrics resource for which metrics are returned, need to be configurable by the users of this package.
• Does the strict handling need to be enabled/disabled by the users of this package.
• Are unit tests needed.

Note, the pinning of asgiref==3.6.0 removes a test error in the py3.8 tox environment (same fix that already existed for the pypy3.8 tox environment). I don't know why the error on py3.8 comes up in the first place. I guess someone more experienced than me needs to look at that.

For details, see the commit message.