Remove flake8-mypy in favor of direct mypy usage

[ddelnano]

Summary: Remove flake8-mypy in favor of direct mypy usage

Our mypy type checking is broken as seen on #2247. The flake8-mypy plugin has been deprecated for a long time, so this replaces it with running mypy directly.

Relevant Issues: N/A

Type of change: /kind dependencies

Test Plan: Verified re-built container works with mypy type checking

$ PX_DOCKER_RUN_AS_ROOT=True ./scripts/run_docker.sh

# Succesful run passes
root@px-dev-docker-dev-vm:/px/src/px.dev/pixie# arc lint src/pxl_scripts/px/collect_heap_dumps.pxl
 OKAY  No lint messages.

# Introduce typing issue and see that `arc lint` notices it
root@px-dev-docker-dev-vm:/px/src/px.dev/pixie# arc lint src/pxl_scripts/px/collect_heap_dumps.pxl
 Exception
Some linters failed:
    - CommandException: Command failed with error #1!
      COMMAND
      mypy --config-file=mypy.ini src/pxl_scripts/px/collect_heap_dumps.pxl

      STDOUT
      src/pxl_scripts/px/collect_heap_dumps.pxl:27:5: error: Unsupported operand types for + ("int" and "str")  [operator]
      Found 1 error in 1 file (checked 1 source file)


      STDERR
      mypy.ini: [mypy]: python_version: Python 3.8 is not supported (must be 3.9 or higher)

(Run with `--trace` for a full exception trace.)