Specify build backend for editable installs

[hugovk]

Fixes the issue that showed up in https://github.com/jazzband/tablib/pull/500, also reproducible locally:

With the old pip==21.2.4 we could do python -m pip install -e . for an editable install.

Now, with new pip==21.3 we need to specify the build backend. See:

• Support editable installs for projects that have a pyproject.toml and use a build backend that supports PEP 660. (#8212)

https://pip.pypa.io/en/stable/news/#features

See also https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/

[codecov[bot]]

Codecov Report

Merging #501 (e25f1df) into master (ebd916e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #501   +/-   ##
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  Coverage   90.78%   90.78%           
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  Files          28       28           
  Lines        2647     2647           
=======================================
  Hits         2403     2403           
  Misses        244      244           

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[claudep]

Thanks!