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In general the patch LGTM, besides few minor comments.
But new implementation uses longer bytecode for annotations. Current code packs argument names in one constant tuple:
$ ./python -m dis
def f(x: int, y: str, z: float): pass
 1 0 LOAD_NAME 0 (int)
 2 LOAD_NAME 1 (str)
 4 LOAD_NAME 2 (float)
 6 LOAD_CONST 0 (('x', 'y', 'z'))
 8 LOAD_CONST 1 (<code object f at 0xb6fc1640, file "<stdin>", line 1>)
 10 LOAD_CONST 2 ('f')
 12 EXTENDED_ARG 4
 14 EXTENDED_ARG 1024
 16 MAKE_FUNCTION 262144
 18 STORE_NAME 3 (f)
 20 LOAD_CONST 3 (None)
 22 RETURN_VALUE
New code uses LOAD_CONST for every name:
$ ./python -m dis
def f(x: int, y: str, z: float): pass
 1 0 LOAD_CONST 0 ('x')
 2 LOAD_NAME 0 (int)
 4 LOAD_CONST 1 ('y')
 6 LOAD_NAME 1 (str)
 8 LOAD_CONST 2 ('z')
 10 LOAD_NAME 2 (float)
 12 BUILD_MAP 3
 14 LOAD_CONST 3 (<code object f at 0xb7035a20, file "<stdin>", line 1>)
 16 LOAD_CONST 4 ('f')
 18 MAKE_FUNCTION 4
 20 STORE_NAME 3 (f)
 22 LOAD_CONST 5 (None)
 24 RETURN_VALUE
With new opcode that creates a dict from values and a tuple of keys (issue27140) new code would be only one instruction longer.