Drop Python 3.7 and dependency refresh

[ajstewart]

• Drops Python 3.7 support.
• Limits upper python to 3.10 (>=3.8, <3.11).
• Poetry lock file refreshed.
• Astropy 4.3 -> 5.0 (Python 3.10 compatibility worked here with jinja).
• pyarrow 3.0 -> 7.0 (Again for Python 3.10 compat).
• Fix collections.Mapping -> collections.abc.Mapping.
• Pinned docs deployment to python 3.8.

Fixes #639.

[ajstewart]

When #642 and #643 are merged, I think the 3.10 test will then pass.