ignore e203 in flake8 to resolve black conflict (#25)

## Summary
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this proposed code change. --->

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## Details
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checkstyle is failing on main because flake8 and black don't agree with
each other due to psf/black#1437. Ignore E203
as black readme suggests.

## Testing Done
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tested. --->

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Complete the following tasks before sending your PR, and replace `[ ]`
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- [ ] run `make test` to ensure correctness
- [x] run `make checkstyle` to ensure code style
```
jobuser [ ~/Liger-Kernel ]$ make checkstyle
flake8 .; flake8_status=$?; \
isort .; isort_status=$?; \
black .; black_status=$?; \
if [ $flake8_status -ne 0 ] || [ $isort_status -ne 0 ] || [ $black_status -ne 0 ]; then \
        exit 1; \
fi
Skipped 1 files
All done! ✨ 🍰 ✨
45 files left unchanged.
```
- [ ] run `make test-convergence` to ensure convergence

.flake8

@@ -5,4 +5,5 @@ exclude =
     .git,
     __pycache__,
     benchmark_internal/others
-extend-ignore=E501,B006,E731,A002
+# E203: https://github.com/psf/black/issues/315
+extend-ignore=E501,B006,E731,A002,E203