Swallow warnings during anonymous compilation of source

[asottile]

Resolves #4260

For the same reasons as this it's difficult to write a future-proof regression test for this.

Here's what I used locally to verify this though:

import pytest


@pytest.mark.xfail
def test_xfail():
    assert False

def test():
    '\d'

before

$ rm -rf __pycache__/ && pytest -Wonce t.py
============================= test session starts ==============================
platform linux -- Python 3.6.6, pytest-3.9.4.dev5+ge6e40db9, py-1.7.0, pluggy-0.8.0
rootdir: /tmp/pytest, inifile: tox.ini
collected 2 items                                                              

t.py x.                                                                  [100%]
=========================== short test summary info ============================
XFAIL t.py::test_xfail

=============================== warnings summary ===============================
t.py:9
  /tmp/pytest/t.py:9: DeprecationWarning: invalid escape sequence \d
    '\d'

t.py::test_xfail
  source:9: DeprecationWarning: invalid escape sequence \d

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=============== 1 passed, 1 xfailed, 2 warnings in 0.05 seconds ================

after

$ rm -rf __pycache__/ && pytest -Wonce t.py
============================= test session starts ==============================
platform linux -- Python 3.6.6, pytest-3.9.4.dev5+ge6e40db9, py-1.7.0, pluggy-0.8.0
rootdir: /tmp/pytest, inifile: tox.ini
collected 2 items                                                              

t.py x.                                                                  [100%]
=========================== short test summary info ============================
XFAIL t.py::test_xfail

=============================== warnings summary ===============================
t.py:9
  /tmp/pytest/t.py:9: DeprecationWarning: invalid escape sequence \d
    '\d'

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=============== 1 passed, 1 xfailed, 1 warnings in 0.05 seconds ================

[codecov]

Codecov Report

Merging #4261 into master will decrease coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #4261      +/-   ##
==========================================
- Coverage   95.87%   95.86%   -0.01%     
==========================================
  Files         109      109              
  Lines       24606    24608       +2     
  Branches     2394     2394              
==========================================
  Hits        23591    23591              
- Misses        720      721       +1     
- Partials      295      296       +1

Flag	Coverage Δ
#docs	29.04% <83.33%> (ø)	⬆️
#doctesting	29.04% <83.33%> (ø)	⬆️
#linting	29.04% <83.33%> (ø)	⬆️
#linux	95.65% <83.33%> (-0.01%)	⬇️
#nobyte	92.02% <83.33%> (ø)	⬆️
#numpy	93.02% <83.33%> (ø)	⬆️
#pexpect	41.66% <83.33%> (ø)	⬆️
#py27	94.05% <83.33%> (ø)	⬆️
#py34	92.34% <83.33%> (+0.03%)	⬆️
#py35	92.35% <83.33%> (+0.03%)	⬆️
#py36	93.99% <83.33%> (-0.01%)	⬇️
#py37	92.37% <83.33%> (+0.03%)	⬆️
#trial	93.02% <83.33%> (ø)	⬆️
#windows	94.13% <83.33%> (-0.01%)	⬇️
#xdist	93.89% <83.33%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/_code/source.py	90.74% <83.33%> (+0.08%)	⬆️
src/_pytest/capture.py	93.39% <0%> (-0.46%)	⬇️

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[nicoddemus]

LGTM, minor nits only. 👍

[blueyed]

Travis finished successfully (https://travis-ci.org/pytest-dev/pytest/builds/447862356) despite the reported (pending) status.