[LINTER] Enforce mypy >= 0.780

Summary:
Found during testing, previous versions are buggy in several ways:

• They don't follow dependencies the same and print the issues from other files
• They have trouble handling python 3.6 type annotation syntax

Test Plan:
Read and try the instructions (tested on Debian 10 and Ubuntu Focal).

arc lint --everything

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9740