[AIRFLOW-2708] unittest2 is reqired for devel, not just devel_ci

[ashb]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2708

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Fix up test-only dep added in [AIRFLOW-1840] Support back-compat on old celery config #3549 that was added to wrong section of setup.py

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: setup.py only change

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[Fokko]

LGTM, thanks

[codecov-io]

Codecov Report

Merging #3581 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3581      +/-   ##
==========================================
- Coverage   76.83%   76.82%   -0.02%     
==========================================
  Files         204      204              
  Lines       15492    15492              
==========================================
- Hits        11903    11901       -2     
- Misses       3589     3591       +2

Impacted Files	Coverage Δ
airflow/jobs.py	82.74% <0%> (-0.27%)	⬇️
airflow/models.py	88.72% <0%> (+0.04%)	⬆️

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