fix(python): Fix _repr_html_ double-height rows (#5645)

[2-5]

Fixes #5645

I've only changed the CSS:

1. removed white-space: pre - the rule causing double-height rows

2. removed scoped from <style scoped> attribute. It was dropped, no current browser implements it - https://stackoverflow.com/a/45692033

3. renamed .dataframe class to .pl-dataframe to prevent interactions with Pandas which also uses .dataframe. The interaction can be observed: add color: red to Polars HTML and you'll see the Pandas output change too.

4. removed th:only-of-type rule, only-of-type is not used by Polars (only Pandas).

5. removed tbody tr th rule, not used by Polars, possibly used by Pandas multi-index.

6. replace descendant selector (thead tr th) with child selector (thead > tr > th) - CSS best practice

7. removed VS Code bug workaround, and line-height: 95% rule.

Note that current Pandas styling is very similar to this PR.

Rows can be made more condensed (less vertical padding) by adding something like below. This is a visual design issue, not for me to decide.

Very condensed (no vertical padding):

.pl-dataframe > tbody > tr > td {
  padding-top: 0;
  padding-bottom: 0,
}

More condensed (less vertical padding):

.pl-dataframe > tbody > tr > td {
  padding-top: 2px;
  padding-bottom: 2px,
}

BEFORE:

AFTER:

[2-5]

Lint Python build failed with github connect error:

error: failed to get `arrow2` as a dependency of package `polars-core v0.26.1 (/home/runner/work/polars/polars/polars/polars-core)`
    ... which satisfies path dependency `polars-core` (locked to 0.26.1) of package `py-polars v0.15.18 (/home/runner/work/polars/polars/py-polars)`

Caused by:
  failed to load source for dependency `arrow2`

Caused by:
  Unable to update https://github.com/ritchie46/arrow2?branch=mmap_slice2#685cf49d

Caused by:
  failed to clone into: /home/runner/.cargo/git/db/arrow2-945af624853845da

Caused by:
  failed to connect to github.com: Connection timed out; class=Os (2)
make: *** [Makefile:38: clippy] Error 101
Error: Process completed with exit code 2.

[stinodego]

Just ran this locally and it looks good to me! Thanks for the contribution.

[alexander-beedie]

Very nice 👍