Disallow untyped defs in synapse._scripts (#12422)

Of note: 

* No untyped defs in `register_new_matrix_user`

This one might be contraversial. `request_registration` has three
dependency-injection arguments used for testing. I'm removing the
injection of the `requests` module and using `unitest.mock.patch` in the
test cases instead.

Doing `reveal_type(requests)` and `reveal_type(requests.get)` before the
change:

```
synapse/_scripts/register_new_matrix_user.py:45: note: Revealed type is "Any"
synapse/_scripts/register_new_matrix_user.py:46: note: Revealed type is "Any"
```

And after:

```
synapse/_scripts/register_new_matrix_user.py:44: note: Revealed type is "types.ModuleType"
synapse/_scripts/register_new_matrix_user.py:45: note: Revealed type is "def (url: Union[builtins.str, builtins.bytes], params: Union[Union[_typeshed.SupportsItems[Union[builtins.str, builtins.bytes, builtins.int, builtins.float], Union[builtins.str, builtins.bytes, builtins.int, builtins.float, typing.Iterable[Union[builtins.str, builtins.bytes, builtins.int, builtins.float]], None]], Tuple[Union[builtins.str, builtins.bytes, builtins.int, builtins.float], Union[builtins.str, builtins.bytes, builtins.int, builtins.float, typing.Iterable[Union[builtins.str, builtins.bytes, builtins.int, builtins.float]], None]], typing.Iterable[Tuple[Union[builtins.str, builtins.bytes, builtins.int, builtins.float], Union[builtins.str, builtins.bytes, builtins.int, builtins.float, typing.Iterable[Union[builtins.str, builtins.bytes, builtins.int, builtins.float]], None]]], builtins.str, builtins.bytes], None] =, data: Union[Any, None] =, headers: Union[Any, None] =, cookies: Union[Any, None] =, files: Union[Any, None] =, auth: Union[Any, None] =, timeout: Union[Any, None] =, allow_redirects: builtins.bool =, proxies: Union[Any, None] =, hooks: Union[Any, None] =, stream: Union[Any, None] =, verify: Union[Any, None] =, cert: Union[Any, None] =, json: Union[Any, None] =) -> requests.models.Response"
```

* Drive-by comment in `synapse.storage.types`

* No untyped defs in `synapse_port_db`

This was by far the most painful. I'm happy to break this up into
smaller pieces for review if it's not managable as-is.