Remove update operator for compatibility, add tests

[quantumjot]

• removes update operator (|=) preventing use on python < 3.9
• adds explicit tests for to_dict methods
• fixes bug with tox config which meant that all tests run on python 3.10

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.15 :tada:

Comparison is base (9b54ccb) 83.92% compared to head (0f5a235) 84.08%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #301 +/- ## ========================================== + Coverage 83.92% 84.08% +0.15% ========================================== Files 30 30 Lines 1904 1904 Branches 295 295 ========================================== + Hits 1598 1601 +3 + Misses 219 216 -3 Partials 87 87 ``` [see 1 file with indirect coverage changes](https://codecov.io/gh/quantumjot/btrack/pull/301/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alan+R+Lowe) 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=Alan+R+Lowe). 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=Alan+R+Lowe)

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[deprecated-napari-hub-preview-bot[bot]]

Preview page for your plugin is ready here: https://preview.napari-hub.org/quantumjot/btrack/301 Updated: 2023-04-17T13:30:34.176732

[quantumjot]

@paddyroddy - I was expecting these new tests to fail on python 3.8 (before I implemented the fix). However they all pass. On closer inspection, it looks like tox is using 3.10 for everything?