python: expose typing information via PEP 561

https://www.python.org/dev/peps/pep-0561/#specification

Create 'py.typed' files in each subpackage that indicate to mypy that
this is a typed module, so that users of any of these packages can use
mypy to check their code as well.

Note: Theoretically it's possible to ditch MANIFEST.in in favor of using
package_data in setup.cfg, but I genuinely could not figure out how to
get it to include things from the *source root* into the *package root*;
only how to include things from each subpackage. I tried!

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-id: 20210629214323.1329806-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/setup.cfg

@@ -19,6 +19,7 @@ classifiers =
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
+    Typing :: Typed
 
 [options]
 python_requires = >= 3.6
@@ -27,6 +28,9 @@ packages =
     qemu.machine
     qemu.utils
 
+[options.package_data]
+* = py.typed
+
 [options.extras_require]
 # Run `pipenv lock --dev` when changing these requirements.
 devel =