net-mgmt/py-ciscoconfparse: Update to 1.5.51

• Limit the Python version to 3.7+ due devel/py-loguru, which requires Python 3.7+ as well.

Changelog since 1.5.30:

https://github.com/mpenning/ciscoconfparse/compare/v1.5.30...v1.5.51

MFH: No (contains mostly feature additions)