TST, COMPAT: Make MMapWrapper tests Windows compatible

[gfyoung]

Partially addresses #13721 by addressing the MMapWrapper test failure. The issue was we didn't have control over the newline character, so the test was re-written to give us that control.

[jreback]

don't u need to close this file (after using it)?

[gfyoung]

Ah, that is true. Will fix.

[jreback]

I think you can use open(..., mode='U')?

[gfyoung]

Unfortunately, that seems to butcher what the end-of-line characters are:

>>> f = open('test.txt', 'w+')
>>> f.write('a\r\n')
3
>>> f.write('a\r\n')
3
>>> f.write('a\r\n')
3
>>> f.close()
>>> f = open('test.txt', mode='U')
>>> f.readlines()   # mmap will recognize the '\r\n' though
['a\n', 'a\n', 'a\n']

[gfyoung]

@jreback : Because this is a readlines issue and NOT an issue with mmap, as workaround, I am going to call .strip() just to remove the newline character and compare actual content.

[codecov-io]

Current coverage is 84.56% (diff: 100%)

Merging #13732 into master will not change coverage

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  Methods           0          0          
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  Hits          43296      43296          
  Misses         7900       7900          
  Partials          0          0          

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[gfyoung]

@jreback : I fixed the test, so Travis now passes. Ready to merge if there are no other concerns.

[jreback]

lgtm (though @jorisvandenbossche if you'd test on windows to be sure)

[jreback]

oddly though I cannot reproduce this anymore.......

[jorisvandenbossche]

Sorry, I wasn't using my windows the last days