[prettier linter] lint cashtab

Summary:
Since the monorepo was re-organized, Cashtab is not linted by prettier

Fix this and lint cashtab with 2.6.0

Test Plan:
arc lint --everything

Note: I ran this, then dropped all the modified py files out of this commit. For whatever reason, arc lint -- cashtab/* wouldn't do anything for me

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13846