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.

include/pybind11/pybind11.h

@@ -238,7 +238,7 @@ protected:
             } else if (c == '}') {
                 // Write default value if available.
                 if (arg_index < rec->args.size() && rec->args[arg_index].descr) {
-                    signature += "=";
+                    signature += " = ";
                     signature += rec->args[arg_index].descr;
                 }
                 arg_index++;