Add spaces around "=" in signature repr.

[anntzer]

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.

[jagerman]

This sounds good to me. PEP8 aside, I also agree that it looks better in most cases.

[jagerman]

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

It seems to me that for some cases (e.g. taking a py::object or py::handle argument) we'd be better off not adding the type annotation. But that's not a concern for this PR.