centralize and de-privatize month/day name constants

[jbrockmendel]

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

[jbrockmendel]

I've got some debugging to do here and in two other places. In the interim, there's a bugfix in #18376 that I'm really eager to get closed.

[codecov]

Codecov Report

Merging #18728 into master will decrease coverage by 0.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master   #18728      +/-   ##
==========================================
- Coverage   91.61%   91.59%   -0.02%     
==========================================
  Files         153      153              
  Lines       51339    51342       +3     
==========================================
- Hits        47034    47028       -6     
- Misses       4305     4314       +9

Flag	Coverage Δ
#multiple	89.46% <90.47%> (ø)	⬆️
#single	40.74% <38.09%> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/tseries/frequencies.py	94.04% <100%> (+0.02%)	⬆️
pandas/tseries/offsets.py	96.95% <88.23%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️

---

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[codecov]

Codecov Report

Merging #18728 into master will decrease coverage by 0.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master   #18728      +/-   ##
==========================================
- Coverage   91.61%   91.59%   -0.02%     
==========================================
  Files         153      153              
  Lines       51361    51364       +3     
==========================================
- Hits        47053    47047       -6     
- Misses       4308     4317       +9

Flag	Coverage Δ
#multiple	89.45% <90.47%> (ø)	⬆️
#single	40.74% <38.09%> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/tseries/frequencies.py	94.04% <100%> (+0.02%)	⬆️
pandas/tseries/offsets.py	96.95% <88.23%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <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 4091f64...a64a31f. Read the comment docs.

[jbrockmendel]

circleCI is a timeout, appveyor looks unrelated, goes through fine locally:

___ TestDataFrameGroupByPlots.test_series_groupby_plotting_nominally_works ____
[gw0] win32 -- Python 3.6.3 C:\Miniconda3_64\envs\pandas\python.exe
self = <pandas.tests.plotting.test_groupby.TestDataFrameGroupByPlots object at 0x000000EA3231CCC0>
    def test_series_groupby_plotting_nominally_works(self):
        n = 10
        weight = Series(np.random.normal(166, 20, size=n))
        height = Series(np.random.normal(60, 10, size=n))
        with tm.RNGContext(42):
            gender = np.random.choice(['male', 'female'], size=n)
    
>       weight.groupby(gender).plot()
pandas\tests\plotting\test_groupby.py:25: 

[mroeschke]

Just a heads up, there are also some day names hardcoded here and here as well.

Linking #18164 as reference for myself.

[jreback]

@jbrockmendel pls add @mroeschke suggestions to the list, whomever wants can do a PR to centralize those as well!