Run lint in parallel in runtest.py (#14448)

Give flake8 3 parallel processes. Previously it was sequential. 

Assuming that most developers have a machine with at least 4 cores, we
have at least one left for self check, which is run in parallel with
lint.

runtests.py

@@ -50,7 +50,7 @@
  # Self type check
  "self": [executable, "-m", "mypy", "--config-file", "mypy_self_check.ini", "-p", "mypy"],
  # Lint
- "lint": ["flake8", "-j0"],
+ "lint": ["flake8", "-j3"],
  "format-black": ["black", "."],
  "format-isort": ["isort", "."],
  # Fast test cases only (this is the bulk of the test suite)