PY_COLORS=1 causes invalid escape codes in --junitxml

Issue: the combination of PY_COLORS=1 and --junitxml causes some weird escaping in the generated xml. Specifically, it seems to partially escape the ANSI escape codes -- converting the ESC into #, but not doing anything else.

Running with just --colors=yes doesn't do this to the generated XML. (This issue really only shows up in my CI environment, where getting things fixed is potentially slow, and could "break" builds for people.) Since it seems like PY_COLORS=1 should mean the same thing as --color=yes I think this is a bug.

This occurs for me with pytest versions 2.8 - 3.1. Pip list for my env with 3.1:

$ pip list
appdirs (1.4.3)
packaging (16.8)
pip (9.0.1)
py (1.4.34)
pyparsing (2.2.0)
pytest (3.1.2)
setuptools (34.3.3)
six (1.10.0)
wheel (0.29.0)

Example:

$ cat test.py
def test_example():
    assert False

############### the bug
$ PY_COLORS=1 pytest --junitxml=junit-env.xml test.py
=============================== test session starts ===============================
platform darwin -- Python 3.5.3, pytest-3.0.6, py-1.4.31, pluggy-0.4.0
rootdir: /Users/bwm/findable/virtualenvs/tmp-203699af9fd9e3ac, inifile:
plugins: cov-2.4.0
collected 1 items

test.py F

 generated xml file: /Users/bwm/findable/virtualenvs/tmp-203699af9fd9e3ac/junit-env.xml
==================================== FAILURES =====================================
__________________________________ test_example ___________________________________

    def test_example():
>       assert False
E       assert False

test.py:2: AssertionError
============================ 1 failed in 0.03 seconds =============================

$ cat junit-env.xml
<?xml version="1.0" encoding="utf-8"?><testsuite errors="0" failures="1" name="pytest" skips="0" tests="1" time="0.028"><testcase classname="test" file="test.py" line="0" name="test_example" time="0.0013718605041503906"><failure message="assert False">#x1B[1m    def test_example():#x1B[0m
#x1B[1m&gt;       assert False#x1B[0m
#x1B[1m#x1B[31mE       assert False#x1B[0m

#x1B[1m#x1B[31mtest.py#x1B[0m:2: AssertionError</failure></testcase></testsuite>%

############## desired behavior
$ pytest --color=yes --junitxml=junit-flag.xml test.py
=============================== test session starts ===============================
platform darwin -- Python 3.5.3, pytest-3.0.6, py-1.4.31, pluggy-0.4.0
rootdir: /Users/bwm/findable/virtualenvs/tmp-203699af9fd9e3ac, inifile:
plugins: cov-2.4.0
collected 1 items

test.py F

 generated xml file: /Users/bwm/findable/virtualenvs/tmp-203699af9fd9e3ac/junit-flag.xml
==================================== FAILURES =====================================
__________________________________ test_example ___________________________________

    def test_example():
>       assert False
E       assert False

test.py:2: AssertionError
============================ 1 failed in 0.03 seconds =============================

$ cat junit-flag.xml
<?xml version="1.0" encoding="utf-8"?><testsuite errors="0" failures="1" name="pytest" skips="0" tests="1" time="0.027"><testcase classname="test" file="test.py" line="0" name="test_example" time="0.0012619495391845703"><failure message="assert False">def test_example():
&gt;       assert False
E       assert False

test.py:2: AssertionError</failure></testcase></testsuite>%

Reproduce the issue with the provided test.py using PY_COLORS=1 pytest --junitxml and compare it with --color=yes. Start at pytest's --junitxml and color-handling entry points, then verify that the generated junit-env.xml contains valid XML text without #x1B escape artifacts while preserving the failure output.