Add stubs for docopt package (#5248)

* Add stubs for `docopt` package

Add the single stub for the simple, but fairly well-used
[docopt](https://github.com/docopt/docopt) package.
It looks like that project is not very well maintained,
so I thought it would be easier to add the type annotations
here, as opposed to there, upstream. This has even been requested a few
times in https://github.com/docopt/docopt/issues/471. It appeared to me
that only the `docopt()` function is the public API, so I didn't include
the rest of the module's functions and attributes.

stubs/docopt/METADATA.toml

@@ -0,0 +1,3 @@
+# Prior to v0.6, docopt() had only 3 optional args
+version = "0.6"
+python2 = true

stubs/docopt/docopt.pyi

@@ -0,0 +1,13 @@
+from typing import Any, Iterable, Optional, Union
+
+__version__: str
+
+_Argv = Union[Iterable[str], str]
+
+def docopt(
+    doc: str,
+    argv: Optional[_Argv] = ...,
+    help: bool = ...,
+    version: Optional[Any] = ...,
+    options_first: bool = ...,
+) -> dict[str, Any]: ...  # Really should be dict[str, Union[str, bool]]