Set long_description_content_type in setup.py

A small change to prevent such warning when publishing wheels:

    Checking dist/watchdog-XXX.whl: PASSED, with warnings
      warning: `long_description_content_type` missing. defaulting to `text/x-rst`.

setup.py

@@ -91,6 +91,7 @@
       version=version.VERSION_STRING,
       description="Filesystem events monitoring",
       long_description=readme + '\n\n' + changelog,
+      long_description_content_type="text/x-rst",
       author="Yesudeep Mangalapilly",
       author_email="[email protected]",
       license="Apache License 2.0",