Force the builtin module key to be the correct type. (#2814)

* Force the builtin module key to be the correct type.

Previously it was always going to be a std::string which converted into
unicode. Python 2 appears to want module keys to be normal str types, so
this was breaking code that expected plain string types in the
builtins.keys() data structure

* Add a simple unit test to ensure all built-in keys are str

* Update the unit test so it will also run on pypy

* Run pre-commit.

Co-authored-by: Jesse Clemens <jesse.clemens@sony.com>

tests/test_modules.py

@@ -75,3 +75,16 @@ def test_duplicate_registration():
     """Registering two things with the same name"""
 
     assert m.duplicate_registration() == []
+
+
+def test_builtin_key_type():
+    """Test that all the keys in the builtin modules have type str.
+
+    Previous versions of pybind11 would add a unicode key in python 2.
+    """
+    if hasattr(__builtins__, "keys"):
+        keys = __builtins__.keys()
+    else:  # this is to make pypy happy since builtins is different there.
+        keys = __builtins__.__dict__.keys()
+
+    assert {type(k) for k in keys} == {str}