ENH (UX): add python-gitlab dependency

[jsheunis]

Fixes #4344

Adds python-gitlab to setup.py so that it does not have to be installed manually when users want to run create-sibling-gitlab

[codecov]

Codecov Report

Merging #5776 (5f90005) into maint (985413a) will decrease coverage by 36.66%.
The diff coverage is 55.55%.

❗ Current head 5f90005 differs from pull request most recent head aa50e9e. Consider uploading reports for the commit aa50e9e to get more accurate results

@@             Coverage Diff             @@
##            maint    #5776       +/-   ##
===========================================
- Coverage   89.89%   53.22%   -36.67%     
===========================================
  Files         299      297        -2     
  Lines       42350    42302       -48     
===========================================
- Hits        38069    22515    -15554     
- Misses       4281    19787    +15506     

Impacted Files	Coverage Δ
datalad/core/distributed/clone.py	89.84% <ø> (-0.26%)	⬇️
datalad/core/distributed/push.py	88.52% <ø> (ø)
datalad/core/local/create.py	92.36% <ø> (-0.70%)	⬇️
datalad/core/local/diff.py	90.62% <ø> (ø)
datalad/core/local/run.py	98.23% <ø> (-0.89%)	⬇️
datalad/core/local/status.py	96.42% <ø> (ø)
datalad/distributed/create_sibling_ria.py	59.41% <ø> (-22.95%)	⬇️
datalad/distributed/export_archive_ora.py	38.46% <ø> (-43.08%)	⬇️
datalad/distribution/create_sibling.py	47.60% <ø> (-38.31%)	⬇️
datalad/distribution/create_sibling_github.py	84.37% <ø> (ø)
... and 243 more

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[mih]

CI failure is unrelated. No need to rerun