Switch mypy approach to pre-commit (#1234)

I'm treating my PR to mypy bazel integration as dead in the water. Abandoning that approach, this seems to work reasonably well as a replacement. It's actually a slightly newer version of mypy too -- the bazel integration was stuck on an old version.

Note I don't think we're losing much: we shouldn't be writing too much python.

migrate_cpp/BUILD

@@ -2,8 +2,6 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-load("@mypy_integration//:mypy.bzl", "mypy_test")
-
 py_binary(
     name = "migrate_cpp",
     srcs = ["migrate_cpp.py"],
@@ -13,9 +11,3 @@ py_binary(
     ],
     python_version = "PY3",
 )
-
-mypy_test(
-    name = "migrate_cpp_mypy_test",
-    include_imports = True,
-    deps = [":migrate_cpp"],
-)