Run mypy type checking with the minimum supported Python version (#15602)

We use the oldest Python version because later Python versions can include some overloads which don't work in the older versions which we still support.

We're using Python 3.8 instead of 3.7 which is our actual minimum support version because it's EOL is in a matter of weeks so can avoid the extra effort. And in any case, minimum Python 3.8 support is better than winging it on Python 3.11.

changelog.d/15602.misc

@@ -0,0 +1 @@
+Run mypy type checking with the minimum supported Python version to catch new usage that isn't backwards-compatible.

mypy.ini

@@ -13,6 +13,9 @@ no_implicit_optional = True
 disallow_untyped_defs = True
 strict_equality = True
 warn_redundant_casts = True
+# Run mypy type checking with the minimum supported Python version to catch new usage
+# that isn't backwards-compatible (types, overloads, etc).
+python_version = 3.8
 
 files =
   docker/,