Added proper Protocol method signature checking

[agronholm]

Changes

Fixes #465.

This adds proper checking of protocol method signatures instead of using check_callable() which does not deal with keyword arguments.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #999, the entry should look like this:

* Fix big bad boo-boo in the pytest plugin (#999 <https://github.com/agronholm/typeguard/issues/999>_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[coveralls]

coverage: 94.076% (-0.1%) from 94.217%
when pulling d344b6e on fix-protocols
into afad2c7 on master.

[coveralls]

coverage: 94.135% (-0.08%) from 94.217%
when pulling a956b00 on fix-protocols
into 0c50de6 on master.

[antonagestam]

I can run the phantom-types test suites against this PR, will try and report back today.

[antonagestam]

phantom-types test suite is passing without issues ✅

[agronholm]

Thanks!