Enable and fix doctests on 2.13

[travisbrown]

This doesn't need to happen before 2.1.0 but we should be running these tests on 2.13.

[codecov-io]

Codecov Report

Merging #3155 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3155      +/-   ##
==========================================
+ Coverage   93.15%   93.18%   +0.02%     
==========================================
  Files         376      376              
  Lines        7353     7353              
  Branches      204      190      -14     
==========================================
+ Hits         6850     6852       +2     
+ Misses        503      501       -2

Flag	Coverage Δ
#scala_version_212	93.52% <ø> (+0.02%)	⬆️
#scala_version_213	92.96% <ø> (+2.12%)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyList.scala	99.35% <ø> (ø)	⬆️
core/src/main/scala/cats/arrow/Choice.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/set.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyVector.scala	99.15% <ø> (ø)	⬆️
core/src/main/scala/cats/NonEmptyTraverse.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptySet.scala	97.64% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/list.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyChain.scala	87.09% <ø> (+1.07%)	⬆️
core/src/main/scala/cats/syntax/flatMap.scala	80.95% <ø> (ø)	⬆️
... and 1 more

---

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 c3eed57...bd17e57. Read the comment docs.

[kailuowang]

Smart way to address the breaking changes in the toString of the new collections.