Remove typing-extensions imports

[tlambert03]

We had some from typing_extensions imports, but didn't have it listed in our dependencies. It's something that ends up being in almost every environment, so we didn't have any issues yet (I guess), but it should have been listed in deps. Turns out though, that everything we were importing there is now available in python >=3.8, so this PR just changes those imports to from typing

[codecov]

Codecov Report

Merging #193 (e7b3c11) into main (2323319) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #193   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines         2106      2101    -5     
=========================================
- Hits          2106      2101    -5     

Impacted Files	Coverage Δ
npe2/_dynamic_plugin.py	100.00% <ø> (ø)
npe2/manifest/utils.py	100.00% <ø> (ø)
npe2/types.py	100.00% <ø> (ø)
npe2/io_utils.py	100.00% <100.00%> (ø)
npe2/manifest/_package_metadata.py	100.00% <100.00%> (ø)
npe2/manifest/contributions/_themes.py	100.00% <100.00%> (ø)

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[nclack]

lgtm