python.tests: add some comments to the tests

pkgs/development/interpreters/python/tests.nix

@@ -1,3 +1,10 @@
+# Tests for the Python interpreters, package sets and environments.
+#
+# Each Python interpreter has a `passthru.tests` which is the attribute set
+# returned by this function. For example, for Python 3 the tests are run with
+#
+# $ nix-build -A python3.tests
+#
 { stdenv
 , python
 , runCommand
@@ -7,6 +14,8 @@
 }:
 
 let
+  # Test whether the interpreter behaves in the different types of environments
+  # we aim to support.
   environmentTests = let
     envs = let
       inherit python;
@@ -83,6 +92,7 @@ let
 
   in lib.mapAttrs testfun envs;
 
+  # Integration tests involving the package set.
   # All PyPy package builds are broken at the moment
   integrationTests = lib.optionalAttrs (python.pythonAtLeast "3.7"  && (!python.isPyPy)) rec {
     # Before the addition of NIX_PYTHONPREFIX mypy was broken with typed packages