doc: Add troubleshooting for Cythonized code and pytest (#293069)

Cython is a Python compiler that emits native .so modules. By default, python derivations run tests in the wrong directory to see these modules and tests fail.

Issue #255262 documents the root cause and solution for this problem.

This PR adds a description of the problem and the most common solution to the test troubleshooting list.

doc/languages-frameworks/python.section.md

@@ -2014,6 +2014,10 @@ example of such a situation is when `py.test` is used.
 
 * Tests that attempt to access `$HOME` can be fixed by using the following
   work-around before running tests (e.g. `preCheck`): `export HOME=$(mktemp -d)`
+* Compiling with Cython causes tests to fail with a `ModuleNotLoadedError`.
+  This can be fixed with two changes in the derivation: 1) replacing `pytest` with
+  `pytestCheckHook` and 2) adding a `preCheck` containing `cd $out` to run
+  tests within the built output.
 
 ## Contributing {#contributing}