[READY] Fix Python 2 support and add Python 3 support on Windows

[micbou]

Python 2 support

We cannot pass an environment variable containing unicode literals to Popen on Windows and Python2. Instead of converting all environment variables to native strings, we only convert the ones we use.

Python 3 support

We update the requests submodule to fix a compatibility error between requests and Python 3.5.

Fix from PR #211 is only needed for Python 2.

In build.py, we prepend the folder of the ycm_core_tests executable to the PATH instead of replacing it so that the executable is able to find the python35.dll library.

We don't reuse Omnisharp instances in C♯ tests on Windows and Python 3 because of the following random error occuring when running those tests:

OSError: [WinError 6] The handle is invalid

AppVeyor configuration

We append the -DUSE_DEV_FLAGS=ON option to EXTRA_CMAKE_ARGS environment variable in run_tests.py script in order to use EXTRA_CMAKE_ARGS when running tests. We need it on AppVeyor to tell CMake the correct path to the Python 3 library according to architecture. Otherwise, it always pick the 64 bits one. I can't reproduce this issue on my machine so I consider it is specific to AppVeyor.

We only do one run for MSVC 11, MSVC 12, and Python 2.7. This reduces the number of runs from 6 to 5.

[Valloric]

Thanks for making sure we continue to work well on Windows! :) Without you, it would probably bitrot.

with minor comments.

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Review status: 0 of 9 files reviewed at latest revision, 2 unresolved discussions.

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build.py, line 3 [r1] (raw file):
Not having this needs a comment explaining why it's missing otherwise someone will add it again in the future.

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build.py, line 283 [r1] (raw file):
Needs comment.

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Comments from the review on Reviewable.io

[micbou]

Reviewed 8 of 9 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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build.py, line 3 [r1] (raw file):
Done.

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build.py, line 283 [r1] (raw file):
Done.

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Comments from the review on Reviewable.io

[Valloric]

Thanks!