Drop Python 3.8 for conda (means dropping for tests) but keep for PyPI.

See #303 for discussion. Essentially conda-forge dropped 3.8 and
the gplately tests first install deps using conda which means the latest
pyGPlates (1.0.0rc1) cannot be installed. So we'll drop 3.8 for conda.
But pyGPlates 1.0.0rc1 still supports 3.8 on PyPI, so GPlately can too.

.github/workflows/build_and_test.yml

@@ -19,7 +19,8 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        # Conda-forge currently supports Python >=3.9 ...
+        python-version: ["3.9", "3.10", "3.11", "3.12"]
 
     defaults:
       run: