ci: downgrade Cutadapt & silence warning

[uniqueg]

• Cap Cutadapt version to <=4.2 to avoid excessive runtime (https://github.com/marcelm/cutadapt/issues/694)
• Use keyword argument n in pandas.Series.str.split() to avoid depreciation warning

Fixes #118 Fixes #120

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (71c6512) 99.78% compared to head (0b41822) 99.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #119 +/- ## ======================================= Coverage 99.78% 99.78% ======================================= Files 12 12 Lines 948 948 ======================================= Hits 946 946 Misses 2 2 ``` | [Impacted Files](https://codecov.io/gh/zavolanlab/htsinfer/pull/119?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zavolanlab) | Coverage Δ | | |---|---|---| | [htsinfer/get\_library\_source.py](https://codecov.io/gh/zavolanlab/htsinfer/pull/119?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zavolanlab#diff-aHRzaW5mZXIvZ2V0X2xpYnJhcnlfc291cmNlLnB5) | `100.00% <ø> (ø)` | | 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=zavolanlab). 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=zavolanlab)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.