Add spaces around "=" in signature repr.

PEP8 indicates (correctly, IMO) that when an annotation is present, the
signature should include spaces around the equal sign, i.e.

    def f(x: int = 1): ...

instead of

    def f(x: int=1): ...

(in the latter case the equal appears to bind to the type, not to the
argument).

pybind11 signatures always includes a type annotation so we can always
add the spaces.

tests/test_stl.py

@@ -164,7 +164,7 @@ def test_stl_pass_by_pointer(msg):
         m.stl_pass_by_pointer()  # default value is `nullptr`
     assert msg(excinfo.value) == """
         stl_pass_by_pointer(): incompatible function arguments. The following argument types are supported:
-            1. (v: List[int]=None) -> List[int]
+            1. (v: List[int] = None) -> List[int]
 
         Invoked with:
     """  # noqa: E501 line too long
@@ -173,7 +173,7 @@ def test_stl_pass_by_pointer(msg):
         m.stl_pass_by_pointer(None)
     assert msg(excinfo.value) == """
         stl_pass_by_pointer(): incompatible function arguments. The following argument types are supported:
-            1. (v: List[int]=None) -> List[int]
+            1. (v: List[int] = None) -> List[int]
 
         Invoked with: None
     """  # noqa: E501 line too long