Fix missing gevent extra dependency in Docker setup

[bmfs]

CATEGORY

Choose one

• Bug Fix

SUMMARY

Created a requirements-extra.txt as was suggested in the issue discussion (#7522)

ADDITIONAL INFORMATION

• Fixes Docker-->ModuleNotFoundError: No module named 'gevent' #7522

[mistercrunch]

Hey @bmfs good to see you here. Looks like you need to add the ASF header on the file to pass the license check...

[codecov-io]

Codecov Report

Merging #7744 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7744      +/-   ##
==========================================
- Coverage   65.78%   65.76%   -0.03%     
==========================================
  Files         459      459              
  Lines       21924    21924              
  Branches     2411     2411              
==========================================
- Hits        14423    14418       -5     
- Misses       7380     7385       +5     
  Partials      121      121

Impacted Files	Coverage Δ
superset/db_engine_specs/postgres.py	72.72% <0%> (-22.73%)	⬇️

---

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 9abcdcc...206b224. Read the comment docs.

[Aylr]

This looks great and I'm looking forward to it being merged!

[aspedrosa]

#7827 added a command to the Dockerfile installing gevent here.
It should be removed with this new update