Drop support for Python 3.7

[crusaderky]

• Drop support for Python 3.7
• Automatically call close() on dereferencing
• Add unit tests for update() and close()

[jrbourbeau]

Thanks @crusaderky!

I pushed a small commit to bump python_requires in setup.cfg, hope you don't mind

[jakirkham]

Thank you both! 🙏

[jrbourbeau]

No problem! Sorry for the lack of response over in #72 -- that had completely dropped off my radar

[jakirkham]

All good. Just happy to make the move generally. Support for more interesting things (like pickle protocol 5, shared memory, etc.) get easier with Python 3.8+