Improve pinning of importlib-metadata for compatability reasons.

Only impacts certain version of Python, where the `flake8` package
is trying to get the entrypoints of a package using a deprecated
method of importlib-metadata. Basically just an issue with the
package dependencies not being defined very well downstream.

pyproject.toml

@@ -127,8 +127,9 @@ deps =
     black
     beautifulsoup4
     dj-database-url
-    importlib-metadata==4.13.0
     factory-boy
+    python3.6: importlib-metadata<5.0  # Needed for compatibility reasons - https://stackoverflow.com/a/73932581
+    python3.7: importlib-metadata<5.0  # Needed for compatibility reasons - https://stackoverflow.com/a/73932581
     django2: Django<3
     django3: Django<4
     django4: Django<5