pythonPackages.mt-940: fix build (#36784)

- added missing `enum34` dependency
- enabled tests
- moved expression into its own file

See ticket #36453
See https://hydra.nixos.org/build/70677609/log

/cc @the-kenny

pkgs/development/python-modules/mt-940/no-coverage.patch

@@ -0,0 +1,26 @@
+diff --git a/pytest.ini b/pytest.ini
+index fef28f5..f366331 100644
+--- a/pytest.ini
++++ b/pytest.ini
+@@ -4,10 +4,6 @@ python_files =
+     tests/*.py
+ 
+ addopts =
+-    --cov mt940
+-    --cov-report term-missing
+-    --cov-report html
+-    --no-cov-on-fail
+     --doctest-modules
+     --pep8
+     --flakes
+diff --git a/tests/requirements.txt b/tests/requirements.txt
+index fc55572..e52cc28 100644
+--- a/tests/requirements.txt
++++ b/tests/requirements.txt
+@@ -3,6 +3,5 @@
+ -r ../docs/requirements.txt
+ pytest
+ pytest-cache
+-pytest-cover
+ pytest-flakes
+ pytest-pep8