zulip_bots: Add custom command for generating MANIFEST.in.

We now have a custom command in zulip_bots/setup.py to generate
a MANIFEST.in. To generate a MANIFEST for a PyPA release, we
can now run:

python setup.py gen_manifest --release

To generate a non-release MANIFEST, we can run:

python setup.py gen_manifest

This allows us to automate the MANIFEST generation in our
release automation script.

tools/provision

@@ -8,6 +8,12 @@ import warnings
 
 from importlib import import_module
 
+CURRENT_DIR = os.path.dirname(os.path.abspath(__file__))
+ZULIP_BOTS_DIR = os.path.join(CURRENT_DIR, '..', 'zulip_bots')
+sys.path.append(ZULIP_BOTS_DIR)
+
+import generate_manifest
+
 def main():
     usage = """./tools/provision