tools: Fix mypy errors with generate-integration-docs-screenshot.
tools: Fix mypy errors with generate-integration-docs-screenshot.

3f4d0f72fd86b2702e1c5324bb37489ffa792332 adjusted the types in
preparation for extending the functionality, but only in later commits
that are not merged yet did it make these updates to the types.

rht/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,andersk/zulip,kou/zulip,zulip/zulip,kou/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip