bump edition to 2021, add msrv 1.56, test coverage

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (573254d) 74.31% compared to head (7a4136e) 79.02%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ========================================== + Coverage 74.31% 79.02% +4.70% ========================================== Files 3 3 Lines 1024 1068 +44 Branches 1024 1068 +44 ========================================== + Hits 761 844 +83 + Misses 194 161 -33 + Partials 69 63 -6 ``` | [Files](https://app.codecov.io/gh/davidhewitt/pythonize/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt) | Coverage Δ | | |---|---|---| | [src/de.rs](https://app.codecov.io/gh/davidhewitt/pythonize/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt#diff-c3JjL2RlLnJz) | `81.81% <100.00%> (+1.39%)` | :arrow_up: | | [src/ser.rs](https://app.codecov.io/gh/davidhewitt/pythonize/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt#diff-c3JjL3Nlci5ycw==) | `89.42% <100.00%> (+9.63%)` | :arrow_up: |

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