Handle changed behaviour of lxml addnext method

[knit-bee]

I removed the two usages of addnext and replaced them by insert. The code is maybe a bit harder to read now, but I thought this would be easier than to check the lxml version or how tails are handled by the addnext method.

I had to adjust two test cases, since some elements now have whitespace instead of None as tail. However, I thought this to be an acceptable change in behaviour.

I tested the changes with lxml==4.9.4 and lxml==5.1.0 and it works with both versions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eec05b2) 97.06% compared to head (e9b33e1) 97.06%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #484   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files          22       22           
  Lines        3404     3408    +4     
=======================================
+ Hits         3304     3308    +4     
  Misses        100      100           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[adbar]

Everything works great, thanks!