CI: Add CI build for Python 3.7

This patch adds Python 3.7 to our CI build matrix.  While we always have
supported Python 3.7, it was an oversight not to test builds with it.
Crucially, now that we use different Cython versions between 3.7 and our
other supported interpreter versions, it is important to check with 3.7
explicitly.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

.github/workflows/build.yaml

@@ -87,6 +87,7 @@ jobs:
     strategy:
       matrix:
         python-version:
+          - "3.7"
           - "3.8"
           - "3.9"
           - "3.10"