Fix new flake8 E741 error from using `l` variable

The just-updated flake8 package hits a bunch of:

    E741 ambiguous variable name 'l'

warnings.  This commit renames them all from `l` to `lst` (they are all
list values) to avoid the error.
diff --git a/tests/test_pytypes.py b/tests/test_pytypes.py
index 94c90a9..992e7fc 100644
--- a/tests/test_pytypes.py
+++ b/tests/test_pytypes.py
@@ -7,11 +7,11 @@
 
 def test_list(capture, doc):
     with capture:
-        l = m.get_list()
-        assert l == ["overwritten"]
+        lst = m.get_list()
+        assert lst == ["overwritten"]
 
-        l.append("value2")
-        m.print_list(l)
+        lst.append("value2")
+        m.print_list(lst)
     assert capture.unordered == """
         Entry at position 0: value
         list item 0: overwritten