add ignore=E203 to flake8

[MikeChen012345]

The line "ignore = E203" is added to the setup.cfg file such that the conflict between flake8 and black can be resolved. The error is raised by flake8 when it detects whitespace before ':' which is against PEP 8 style guide recommendations for slices.
However, there's a known discrepancy between flake8 and black regarding this specific rule. black formats code with whitespace before and after the colon in slices for better readability, while flake8 raises a warning for this.

[borjasotomayor]

This looks good to me but, for future reference, can you provide a link to where this is called out as a "known discrepancy"?

[borjasotomayor]

Please also provide more details on why W=503 was added (you can just include the explanation you provided on Slack)

[MikeChen012345]

The W503 warning is raised by flake8 when it detects a line break before a binary operator. This was once against PEP 8 recommendations, which suggested that line breaks should occur after the binary operator.
However, PEP 8 was updated and now recommends that line breaks should occur before the binary operator. This makes the code more readable.

[MikeChen012345]

The E203 error was discussed in a bunch of issues. Below are two of them:
PyCQA/pycodestyle#373
psf/black#280

And the W503 error was discussed here:
https://www.flake8rules.com/rules/W503.html

[borjasotomayor]

Thanks for the additional information. This change looks good to me.

[elizabethli31]

Looks good! Please update with dev and I'll merge.

[elizabethli31]

Issue Score: Excellent

Comments:
Good job flagging and fixing this! General change still needs a closing statement.