Create Python tools in favour of shell scripts

Python is likely to be installed when working on Python code,
which should make it more portable and consistent.

tools/codegen.py

@@ -0,0 +1,24 @@
+"""
+Run `telethon_generator.codegen` on both `api.tl` and `mtproto.tl` to output
+corresponding Python code in the default directories under the `client/`.
+"""
+import subprocess
+import sys
+
+GENERATOR = "telethon_generator.codegen"
+ROOT = "client/src/telethon/_impl"
+
+
+def run(*args: str) -> int:
+    return subprocess.run((sys.executable, "-m", *args)).returncode
+
+
+def main() -> None:
+    exit(
+        run(GENERATOR, "api.tl", f"{ROOT}/tl")
+        or run(GENERATOR, "mtproto.tl", f"{ROOT}/tl/mtproto")
+    )
+
+
+if __name__ == "__main__":
+    main()