Drop Python 3.7 support

[odashi]

Description

This is the reminder issue to drop Python 3.7 support from this library.
Since 3.7 is alive until 2023-06-27, we need to process this issue after such date.

3.7 involves many syntax incompatibility with newer versions. It is sometimes frustrating to add/maintain the feature that requires a context switch with sys.version_info.minor.

Additional context

The EOL timeline: https://devguide.python.org/versions/