Fix missing images on PyPI page

[krz]

I replaced the image links in the README.md with their absolute paths. This should fix issue #261 . The codecov link is unchanged and will still not be displayed properly on PyPi (probably because of the required token id).

[codecov[bot]]

Codecov Report

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

Comparison is base (c3c542d) 75.91% compared to head (33dea09) 75.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #270 +/- ## ======================================= Coverage 75.91% 75.91% ======================================= Files 20 20 Lines 1308 1308 ======================================= Hits 993 993 Misses 315 315 ```

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[twiecki]

Thanks @krz!