BLD: Include tslibs src

[TomAugspurger]

Fixes the pip build in pandas-dev/pandas-ci. This worked for me locally in an env setup with https://github.com/pandas-dev/pandas-release.

Closes #22192

[codecov]

Codecov Report

Merging #24664 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24664      +/-   ##
==========================================
- Coverage   92.37%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52315    52315              
==========================================
- Hits        48328    48327       -1     
- Misses       3987     3988       +1

Flag	Coverage Δ
#multiple	90.79% <ø> (ø)	⬆️
#single	43.06% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	88% <0%> (-0.1%)	⬇️

---

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[codecov]

Codecov Report

Merging #24664 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24664      +/-   ##
==========================================
- Coverage   92.37%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52315    52315              
==========================================
- Hits        48328    48327       -1     
- Misses       3987     3988       +1

Flag	Coverage Δ
#multiple	90.79% <ø> (ø)	⬆️
#single	43.06% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	88% <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 1ae466c...f45c47a. Read the comment docs.

[TomAugspurger]

The build log looks OK. Going to merge this and trigger a build in pandas-ci.

[TomAugspurger]

https://travis-ci.org/pandas-dev/pandas-ci/jobs/476662030 passed for the pip-build.

The conda job is failing. Haven't looked closely, but it may be running the 0.23.4 test suite.