DOC GH15643 Removed pytest-xdist from requirements_dev.txt file

[linebp]

• [ x] closes pytest-xdist not found by conda #15643
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

[codecov-io]

Codecov Report

Merging #15646 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15646      +/-   ##
==========================================
- Coverage   91.02%      91%   -0.03%     
==========================================
  Files         143      143              
  Lines       49324    49324              
==========================================
- Hits        44898    44886      -12     
- Misses       4426     4438      +12

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.76% <0%> (-0.35%)	❌
pandas/core/common.py	90.96% <0%> (-0.34%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a703abc...4183f74. Read the comment docs.

[jreback]

thanks!