Point mypy at redwood type stub directly

Instead of the possibly slow process of compiling redwood and installing
it into the virtualenv, we can just point mypy at the stub directly.
(For some reason mypy wanted us to use an explicit "tuple" type.)

Notably this means you can run the lint process without needing a Rust
toolchain installed.

redwood/redwood/__init__.pyi

@@ -3,7 +3,7 @@
 from pathlib import Path
 from typing import BinaryIO
 
-def generate_source_key_pair(passphrase: str, email: str) -> (str, str, str): ...
+def generate_source_key_pair(passphrase: str, email: str) -> tuple[str, str, str]: ...
 def encrypt_message(recipients: list[str], plaintext: str, destination: Path) -> None: ...
 def encrypt_stream(recipients: list[str], plaintext: BinaryIO, destination: Path) -> None: ...
 def decrypt(ciphertext: bytes, secret_key: str, passphrase: str) -> bytes: ...

securedrop/bin/run-mypy

@@ -7,8 +7,9 @@ REPOROOT=$(git rev-parse --show-toplevel)
 cd "${REPOROOT}"
 
 if [ "$(command -v mypy)" ]; then
-    source "securedrop/bin/dev-deps"
-    build_redwood
+    # Point mypy at the redwood package with type stub, see second option on
+    # <https://mypy.readthedocs.io/en/stable/stubs.html#creating-a-stub>.
+    export MYPYPATH="./redwood"
     mypy ./securedrop ./admin --namespace-packages --explicit-package-bases "$@"
 elif [ -d "/opt/venvs/securedrop-app-code/" ]; then
     # Inside the dev container, but no mypy