Fix pip version hack in gdocs sync GH action

[melange396]

https://github.com/cmu-delphi/delphi-epidata/pull/1503 put a hack into the update_gdocs_data.yml GH action to fix a problem that appeared because of a pip version change. Resolve this in a more proper way.

[dshemetov]

• AFAICT, we can probably replace pip install -r requirements.dev.txt in that action with just pip install invoke requests, since it only needs to run tasks.py
• the dependency holding back pip is dropbox; I wasn't able to figure out where pip is pinned in that package config, so I can't tell if version 12.0.0 changed that; might be easiest to just test it
• the only file that needs that dependency is cdc_dropbox_receiver; not sure if that acquisition code runs anymore, but regardless, this makes me wish for isolated envs for acquisition pipelines (related #939)