FIX: fake out setuptools scm in tox on ci

The repository in GHA is a shallow checkout (so to tags) so we are going to get
the version wrong no matter what.  However, a recent change to our build
dependencies (we think meson-python at 0.17) causes setumtools scm to fail to
extract any version when installing from a generated sdist.

By setting a pretend version setuptools_scm will never try to look at git.  It
is not yet clear if this fixes the sdist itself or just continues to use the
pretend version when installing.

Author: tacaswell

tox.ini

@@ -12,6 +12,8 @@ setenv =
     MPLCONFIGDIR={envtmpdir}/.matplotlib
     PIP_USER = 0
     PIP_ISOLATED = 1
+    SETUPTOOLS_SCM_PRETEND_VERSION_FOR_MATPLOTLIB = 0.0.0
+
 usedevelop = True
 commands =
     pytest --pyargs matplotlib.tests  {posargs}