Update pyproject.toml

[lewisblake]

0.21.0.dev0

Change Summary

update pyproject.toml to 0.21.0.dev0

Related issue number

NA, Heiko message in chat

Checklist

• [x] Start with a draft-PR
• [x] The PR title is a good summary of the changes
• [x] PR is set to AeroTools and a tentative milestone
• [x] Documentation reflects the changes where applicable
• [x] Tests for the changes exist where applicable
• [x] Tests pass locally
• [x] Tests pass on CI
• [x] At least 1 reviewer is selected
• [x] Make PR ready to review

[codecov[bot]]

Codecov Report

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

Project coverage is 79.33%. Comparing base (12ffff3) to head (d95899c). Report is 1 commits behind head on main-dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main-dev #1239 +/- ## ========================================= Coverage 79.33% 79.33% ========================================= Files 133 133 Lines 20284 20284 ========================================= Hits 16092 16092 Misses 4192 4192 ``` | [Flag](https://app.codecov.io/gh/metno/pyaerocom/pull/1239/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/metno/pyaerocom/pull/1239/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | `79.33% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno#carryforward-flags-in-the-pull-request-comment) to find out more.

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