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.

Author: JukkaL

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)