Add types stub as requirement

This should address the mypy errors
diff --git a/server/requirements.txt b/server/requirements.txt
index adc8ed8..15db9cd 100644
--- a/server/requirements.txt
+++ b/server/requirements.txt
@@ -1,6 +1,7 @@
 # Items in this file must have a licence compatible with AL 2.0
 aiohttp~=3.6.2                   # AL2.0
 PyYAML~=5.3.1                    # WTFPL
+types-PyYAML                     # AL2.0
 multipart~=0.2.1                 # MIT
 elasticsearch-dsl>=7.0.0,<8.0.0  # AL2.0
 elasticsearch[async]~=7.13.1     # AL2.0