Rename retworkx to rustworkx

[quantumjim]

I just did a find and replace with the hopes of fixing #62.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a58fe36) 100.00% compared to head (8b31bfb) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #63 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 3 3 Lines 331 331 ========================================= Hits 331 331 ``` | [Impacted Files](https://codecov.io/gh/oscarhiggott/PyMatching/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/pymatching/matching.py](https://codecov.io/gh/oscarhiggott/PyMatching/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3B5bWF0Y2hpbmcvbWF0Y2hpbmcucHk=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[oscarhiggott]

Thanks for the PR! Aside from the tests failing, unfortunately I can't merge it as a find-replace will result in backward incompatibility (see my comment in #62)