Add package pre-commit version 2.20.0

[taldcroft]

Add package pre-commit version 2.20.0 from conda-forge. This brings a few new dependencies:

  Package       Version  Build           Channel                  Size
────────────────────────────────────────────────────────────────────────
  Install:
────────────────────────────────────────────────────────────────────────

  + cfgv          3.3.1  pyhd8ed1ab_0    conda-forge/noarch      11 KB
  + distlib       0.3.4  pyhd8ed1ab_0    conda-forge/noarch     371 KB
  + identify      2.5.1  pyhd8ed1ab_0    conda-forge/noarch      77 KB
  + nodeenv       1.7.0  pyhd8ed1ab_0    conda-forge/noarch      32 KB
  + pre-commit   2.20.0  py38h50d1736_0  conda-forge/osx-64     233 KB
  + ukkonen       1.0.1  py38h12bbefe_1  conda-forge/osx-64      11 KB
  + virtualenv  20.15.1  py38h50d1736_0  conda-forge/osx-64       9 MB

  Summary:

  Install: 7 packages

  Total download: 10 MB

[taldcroft]

Including this by default might be controversial since it is purely for development. But maybe we don't care about another 10 Mb.