Add support for pep 660 editable installs in extension

[ericpre]

To allow using pyproject.toml based builds in extension and simplify packaging of the split extension.
Relevant documentation:

• https://peps.python.org/pep-0610/#example-pip-commands-and-their-effect-on-direct-url-json
• https://peps.python.org/pep-0660/#frontend-requirements

Progress of the PR

• Add support for pep660 on editable installs for pyproject.toml based builds of extension,
• [n/a] update docstring (if appropriate),
• [n/a] update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• [n/a] add tests,
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9ecef39) 80.84% compared to head (9eb4983) 80.78%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3252      +/-   ##
======================================================
- Coverage               80.84%   80.78%   -0.06%     
======================================================
  Files                     140      140              
  Lines                   20361    20364       +3     
  Branches                 4821     4820       -1     
======================================================
- Hits                    16460    16451       -9     
- Misses                   2827     2840      +13     
+ Partials                 1074     1073       -1     

Files	Coverage Δ
hyperspy/extensions.py	75.00% <100.00%> (+2.27%)	⬆️

... and 2 files with indirect coverage changes

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[CSSFrancis]

@ericpre So if I understand this correctly this should better handle installing hyperspy and extension packages in the development mode using pip?

[ericpre]

Yes, but only for extensions using pyproject.toml to define the package metadata. See for example: the packaging in hyperspy/exspy#10 will not work without this PR. This PR allows more flexibility in the packaging of extensions. This is only necessary for editable installation (pip -e), so it will not affect users but it still needs to work for developers too! 😅

[ericpre]

Considering the scope of this changes, I don't think that this is worth testing on CI, because it is something that will very rarely.
However, the behaviour needs to be tested manually now on various platform, I have tested on windows and linux but it would be good if someone else test it too. It simply need to install this branch of exspy and checked if the signal registration mechanism works, etc. by running the test suite, for example.

[ericpre]

@CSSFrancis, did you have to check it?

[CSSFrancis]

@CSSFrancis, did you have to check it?

Sorry, I've been out of the office the last couple of days. I'll check it tonight! I have an M1 Mac that I can try

[CSSFrancis]

@ericpre This appears to work on an M1 Mac as well.

[ericpre]

Thanks @CSSFrancis!