Drop unused Python 3.7 compatibility code

[jakirkham]

Now that Python 3.8 is the minimum supported version, there is no need for this typing_extensions branch or a dependency on typing-extensions. So drop both to cleanup the code. Also should bring coverage back up.

xref: #405

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

Merging #406 (1237569) into main (4a029c9) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main      #406      +/-   ##
===========================================
+ Coverage   99.95%   100.00%   +0.04%     
===========================================
  Files          54        54              
  Lines        2093      2089       -4     
===========================================
- Hits         2092      2089       -3     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
numcodecs/ndarray_like.py	100.00% <100.00%> (+2.38%)	⬆️