BLD: Require enum34 for Python < 3.4

[effigies]

#22802 imports enum, which was added to the built-ins in Python 3.4. The enum34 package backports enum to earlier Python versions, so this should be a dependency as long as Python 2.7 is still supported.

To demonstrate the issue, creating a minimal environment using conda and installing the nightly Pandas build suffices:

conda create -y -n pandas_nightly python=2.7
conda activate pandas_nightly
pip install --pre --find-links=https://5cf40426d9f06eb7461d-6fe47d9331aba7cd62fc36c7196769e4.ssl.cf2.rackcdn.com --find-links https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com pandas

Then:

$ python -c 'import pandas'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/anaconda3/envs/pandas_nightly/lib/python2.7/site-packages/pandas/__init__.py", line 35, in <module>
    "the C extensions first.".format(module))
ImportError: C extension: No module named enum not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.

If I install enum34:

$ pip install enum34
Collecting enum34
  Using cached https://files.pythonhosted.org/packages/c5/db/e56e6b4bbac7c4a06de1c50de6fe1ef3810018ae11732a50f15f62c7d050/enum34-1.1.6-py2-none-any.whl
Installing collected packages: enum34
Successfully installed enum34-1.1.6
$ python -c 'import pandas'
$

• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Not sure what the best way to test this would be. Currently, installing hypothesis masks this error, because it contains the requirement enum34; python_version == "2.7".

Related: MacPython/pandas-wheels#36, bids-standard/pybids#276, bids-standard/pybids#315

[pep8speaks]

Hello @effigies! Thanks for submitting the PR.

• There are no PEP8 issues in the file setup.py !

[codecov]

Codecov Report

Merging #24153 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24153      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
- Hits        47671    47670       -1     
- Misses       4029     4030       +1

Flag	Coverage Δ
#multiple	90.6% <ø> (-0.01%)	⬇️
#single	43.02% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/internals/blocks.py	93.65% <0%> (-0.07%)	⬇️

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[codecov]

Codecov Report

Merging #24153 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24153      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
- Hits        47671    47670       -1     
- Misses       4029     4030       +1

Flag	Coverage Δ
#multiple	90.6% <ø> (-0.01%)	⬇️
#single	43.02% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/internals/blocks.py	93.65% <0%> (-0.07%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[datapythonista]

I guess this makes sense, even if after the next version only Python>=3.5 will be supported, so it'll be only for 0.24.

@TomAugspurger

[jreback]

we may actually want to just change it and not use enums which is slightly simpler

adding a required dependency has some implications for wheel and conda building (meaning we haven’t touched these in a while)

[TomAugspurger]

Yeah we should be able to write this without an enum fairly easily.

[jreback]

superseded by #24170