fetch-docs: Add User-Agent to session

Adding User-Agent as an attempt to fix the 403 Client Errors returned
from requests to GitHub.

src/fetch-docs.py

@@ -27,6 +27,9 @@
 if __name__ == '__main__':
     # Use a Session for connection pooling
     session = requests.Session()
+    session.headers.update({
+        "User-Agent": "https://github.com/nextstrain/docs.nextstrain.org ([email protected])",
+    })
 
     class RemoteDoc:
         def __init__(self, source_url, dest_path):