Merge pull request #709 from sed-i/feature/typing

#709

When running static checks, mypy complains:

```
tests/integration/test_bundle.py:24: error: Skipping analyzing "juju": module is installed, but missing library stubs or py.typed marker [import]
 import juju
 ^
```

Error goes away after introducing the py.typed marker.

Ref: [Mypy docs for PEP-516](https://mypy.readthedocs.io/en/latest/installed_packages.html#creating-pep-561-compatible-packages)

Author: jujubot

juju/py.typed

@@ -30,6 +30,7 @@
     version=version.read_text().strip(),
     packages=find_packages(
         exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
+    package_data={'juju': ['py.typed']},
     install_requires=[
         'macaroonbakery>=1.1,<2.0',
         'pyRFC3339>=1.0,<2.0',