Make numexpr an optional dependency to allow installation in pyodide

[ericpre]

Progress of the PR

• make numexpr an optional dependency,
• update installation instruction accordingly,
• fix heading in user guide
• [n/a] docstring updated (if appropriate),
• update user guide (if appropriate),
• update tests,
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:exspy build of this PR (link in github checks)
• ready for review.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.63%. Comparing base (75ca046) to head (7775257).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
exspy/_utils.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #80   +/-   ##
=======================================
  Coverage   88.62%   88.63%           
=======================================
  Files          76       77    +1     
  Lines        7411     7425   +14     
  Branches     1192     1195    +3     
=======================================
+ Hits         6568     6581   +13     
  Misses        574      574           
- Partials      269      270    +1     

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[ericpre]

@francisco-dlp, this should allow installation in pyodide.