Specify pip build backend

[philippjfr]

Fixes #1995

[codecov]

Codecov Report

Merging #2151 (79dcf10) into master (56ce69c) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2151      +/-   ##
==========================================
- Coverage   83.81%   83.45%   -0.36%     
==========================================
  Files         182      182              
  Lines       21672    21233     -439     
==========================================
- Hits        18165    17721     -444     
- Misses       3507     3512       +5     

Impacted Files	Coverage Δ
panel/tests/pane/test_base.py	87.09% <0.00%> (-12.91%)	⬇️
panel/config.py	46.15% <0.00%> (-5.79%)	⬇️
panel/auth.py	36.21% <0.00%> (-3.30%)	⬇️
panel/pane/ipywidget.py	47.05% <0.00%> (-2.95%)	⬇️
panel/io/reload.py	63.95% <0.00%> (-2.72%)	⬇️
panel/io/callbacks.py	54.16% <0.00%> (-2.42%)	⬇️
panel/tests/template/test_manual.py	60.63% <0.00%> (-2.37%)	⬇️
panel/io/state.py	58.42% <0.00%> (-2.28%)	⬇️
panel/pane/plot.py	54.33% <0.00%> (-2.26%)	⬇️
panel/pane/streamz.py	65.71% <0.00%> (-1.86%)	⬇️
... and 82 more

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