Drop python 3.7

[ericpre]

Progress of the PR

• Drop python 3.7,
• update minimum version of dependencies and simplify code accordingly,
• [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

Patch coverage: 100.00% and project coverage change: +0.11 🎉

Comparison is base (e9461e1) 80.67% compared to head (7a5b1f1) 80.79%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3144      +/-   ##
======================================================
+ Coverage               80.67%   80.79%   +0.11%     
======================================================
  Files                     176      176              
  Lines                   24512    24479      -33     
  Branches                 5713     5707       -6     
======================================================
+ Hits                    19775    19777       +2     
+ Misses                   3402     3382      -20     
+ Partials                 1335     1320      -15     

Impacted Files	Coverage Δ
hyperspy/_components/error_function.py	100.00% <ø> (+18.18%)	⬆️
hyperspy/_components/skew_normal.py	91.00% <ø> (+1.57%)	⬆️
hyperspy/_components/voigt.py	93.10% <ø> (+2.78%)	⬆️
hyperspy/learn/svd_pca.py	95.71% <ø> (ø)
hyperspy/model.py	86.73% <ø> (+0.12%)	⬆️
hyperspy/_components/power_law.py	97.22% <100.00%> (-0.08%)	⬇️
hyperspy/_signals/lazy.py	90.86% <100.00%> (+0.94%)	⬆️
hyperspy/learn/mva.py	82.91% <100.00%> (-0.02%)	⬇️
hyperspy/misc/utils.py	91.53% <100.00%> (+0.07%)	⬆️
hyperspy/signal.py	77.73% <100.00%> (+0.10%)	⬆️

... and 5 files with indirect coverage changes

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[CSSFrancis]

Looks good to me!