Drop Python 3.7 through 3.9 support

[ivanovmg]

As discussed in PR #495, it is necessary to drop support for all Python versions up to 3.10. It means that we should drop support for Python 3.7, 3.8 and 3.9.

          Drop all pythons up to 3.10, just like we do in `returns`

Originally posted by @sobolevn in https://github.com/dry-python/classes/issues/495#issuecomment-2541001220