chore(deps): update dependency pypandoc to v1.6.4 (#990)

Author: renovate-bot

packages/gapic-generator/requirements.txt

@@ -4,6 +4,6 @@ googleapis-common-protos==1.53.0
 jinja2==3.0.1
 MarkupSafe==2.0.1
 protobuf==3.17.3
-pypandoc==1.6.3
+pypandoc==1.6.4
 PyYAML==5.4.1
 dataclasses==0.6  # TODO(busunkim) remove when 3.6 support is dropped