Reduce the number of executed local imports (#11545)

This should speed things up a little, because mypyc
is bad at compiling local imports (mypyc/mypyc#903).

mypy/argmap.py

@@ -168,11 +168,10 @@ def expand_actual_type(self,
         This is supposed to be called for each formal, in order. Call multiple times per
         formal if multiple actuals map to a formal.
         """
-        from mypy.subtypes import is_subtype
-
         actual_type = get_proper_type(actual_type)
         if actual_kind == nodes.ARG_STAR:
             if isinstance(actual_type, Instance) and actual_type.args:
+                from mypy.subtypes import is_subtype
                 if is_subtype(actual_type, self.context.iterable_type):
                     return map_instance_to_supertype(
                         actual_type,
@@ -195,6 +194,7 @@ def expand_actual_type(self,
             else:
                 return AnyType(TypeOfAny.from_error)
         elif actual_kind == nodes.ARG_STAR2:
+            from mypy.subtypes import is_subtype
             if isinstance(actual_type, TypedDictType):
                 if formal_kind != nodes.ARG_STAR2 and formal_name in actual_type.items:
                     # Lookup type based on keyword argument name.

mypy/typeops.py

@@ -209,8 +209,6 @@ class B(A): pass
     b = B().copy()  # type: B
 
     """
-    from mypy.infer import infer_type_arguments
-
     if isinstance(method, Overloaded):
         return cast(F, Overloaded([bind_self(c, original_type, is_classmethod)
                                    for c in method.items]))
@@ -230,6 +228,8 @@ class B(A): pass
 
     variables: Sequence[TypeVarLikeType] = []
     if func.variables and supported_self_type(self_param_type):
+        from mypy.infer import infer_type_arguments
+
         if original_type is None:
             # TODO: type check method override (see #7861).
             original_type = erase_to_bound(self_param_type)