loader: import `argparse`
loader: import `argparse`

Summary:
When the `except argparse.ArgumentError` case is hit, the code throws
anyway because the `argparse` identifier is not bound.

Test Plan:
Running `flake8 --select=F82` to check for name binding violations now
reports the whole repository as clean; previously, it caught this error.

wchargin-branch: import-argparse
wchargin-source: 9824edfa4b507e50ba5faa37b4cebd0bb57c5542

PAIR-code/what-if-tool,pair-code/what-if-tool,PAIR-code/what-if-tool,pair-code/what-if-tool,PAIR-code/what-if-tool,PAIR-code/what-if-tool,pair-code/what-if-tool,pair-code/what-if-tool,pair-code/what-if-tool,PAIR-code/what-if-tool