Fix #7445 Use double quotes in error messages

[PrasanthChettri]

Description

Fixes #7445
Replaces all the instances where single quotes are used in messages with double quotes around the mpypy/messages.py

Test Plan

Only some minor changes in one file, have not done tests, pytest seems to freeze when running pytest -q mypy(not a hardware constraint I can assure), would be helpful if anybody could help me with this

[PrasanthChettri]

Im getting an Invalid Ouput Assertion error during testing

Detail

Alignment of first line difference:
  E: ...note: Revealed type is 'Any'
  A: ...note: Revealed type is "Any"

From what I gather this is the intended output we want i.e. error messages with double quotes but the tests given below are failing because of it. If anybody would give a feedback it will be helpful