Stop running CI under Python 3.7 (#69)

The Python 3.7 CI has been problematic for a while now, as the
integration tests tend to crash for unclear reasons. Given that this
does not appear under 3.8, 3.9 or 3.10, and that 3.7 is now past it's
end-of-life, in this PR I drop the 3.7 CI.

.github/workflows/ci.yml

@@ -14,8 +14,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - environment-file: environment-py37.yml
-            build-name: "python 3.7.7, tf 2.1.0, rdkit 2020.09.1"
           - environment-file: environment-py38.yml
             build-name: "python 3.8.16, tf 2.6.2, rdkit 2021.09.1"
           - environment-file: environment-py39.yml