drop python 3.7 support

[davidism]

Python 3.7 is EOL. Set minimum version to 3.8, and remove compatibility code for lower versions.

[jeffwidman]

We'd left this in a while back even after EOL because we didn't think it cost us much tech debt to leave it in, but this PR shows there's a lot of little things to cleanup, so I'm personally 👍 on it.

This will also unlock:

• Drop unnecessary Blinker dependency #248

[davidism]

Even if there aren't significant changes, supporting old versions still has a contribution/maintenance cost, as CI might go away, it might be hard to install an old version, contributors might have to understand differences they typically don't deal with, etc. Since we use python_requires metadata, old Pythons will continue to install the last supported version.