Fix mypy type checking (#184)

A [customer requested][original-pr] that we start including a py.typed file in our
repository. This would enable mypy to take advantage of our typehints.
Unfortunately, this didn't completely solve the customers issue.

A [second pr][second-pr] was opened to address the missing step of including the
py.typed file in the `Manifest.in` file. However, this change alone is not
sufficient.

According to the [documentation][include_package_data], you must also
include the `include_package_data=True` directive so that files
specified in the `Manifest.in` file are included in distribution.

[original-pr]: #166
[second-pr]: #172
[include_package_data]: https://setuptools.pypa.io/en/latest/userguide/datafiles.html#include-package-data

MANIFEST.in

@@ -4,3 +4,4 @@ include test-requirements.txt
 include consul-requirements.txt
 include dynamodb-requirements.txt
 include redis-requirements.txt
+include ldclient/py.typed

setup.py

@@ -54,6 +54,7 @@ def run(self):
     author='LaunchDarkly',
     author_email='[email protected]',
     packages=find_packages(),
+    include_package_data=True,
     url='https://github.com/launchdarkly/python-server-sdk',
     description='LaunchDarkly SDK for Python',
     long_description='LaunchDarkly SDK for Python',