Editable mode is now mandatory

[sbidoul]

Now that PEP 660 is a thing and is supported by pip and flit at least, make editable install non-optional.

In retrospect, installing in non-editable mode was a bad UX anyway.

[codecov]

Codecov Report

Merging #65 (43cd638) into master (b7798b5) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   95.60%   95.33%   -0.27%     
==========================================
  Files          16       15       -1     
  Lines         932      921      -11     
  Branches      173      170       -3     
==========================================
- Hits          891      878      -13     
- Misses         27       28       +1     
- Partials       14       15       +1     

Impacted Files	Coverage Δ
src/pip_deepfreeze/__main__.py	88.88% <ø> (-1.81%)	⬇️
src/pip_deepfreeze/sync.py	92.59% <ø> (ø)
src/pip_deepfreeze/pip.py	96.15% <100.00%> (-1.95%)	⬇️

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