Show deprecation message when running under Python 2.7 and 3.4

[nicoddemus]

Output:

testing\deprecated_test.py .                                             [100%]

========================== deprecated python version ==========================
You are using Python 2.7.15, which will no longer be supported in pytest 5.0
For more information, please read:
  https://docs.pytest.org/en/latest/py27-py34-deprecation.html
=================== 1 passed, 14 deselected in 0.13 seconds ===================

Fix #4627

[codecov]

Codecov Report

Merging #4690 into features will increase coverage by 0.21%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #4690      +/-   ##
============================================
+ Coverage      95.5%   95.71%   +0.21%     
============================================
  Files           111      111              
  Lines         24789    25045     +256     
  Branches       2469     2492      +23     
============================================
+ Hits          23674    23973     +299     
+ Misses          784      755      -29     
+ Partials        331      317      -14

Flag	Coverage Δ
#docs	29.42% <11.11%> (-0.11%)	⬇️
#doctesting	29.42% <11.11%> (-0.11%)	⬇️
#linting	29.42% <11.11%> (-0.11%)	⬇️
#linux	95.55% <100%> (+0.04%)	⬆️
#nobyte	92.29% <88.88%> (+0.6%)	⬆️
#numpy	93.12% <88.88%> (+51.02%)	⬆️
#pexpect	42.64% <44.44%> (+0.55%)	⬆️
#py27	93.17% <88.88%> (-0.4%)	⬇️
#py34	91.79% <88.88%> (+0.16%)	⬆️
#py35	91.79% <55.55%> (?)
#py36	91.81% <55.55%> (+0.18%)	⬆️
#py37	93.33% <55.55%> (-0.36%)	⬇️
#trial	93.12% <88.88%> (+51.02%)	⬆️
#windows	93.87% <100%> (?)
#xdist	93.39% <100%> (-0.19%)	⬇️

Impacted Files	Coverage Δ
testing/deprecated_test.py	100% <100%> (ø)	⬆️
testing/acceptance_test.py	97.84% <100%> (+1.08%)	⬆️
src/_pytest/terminal.py	91.98% <100%> (+0.95%)	⬆️
testing/test_pdb.py	98.36% <0%> (-0.18%)	⬇️
src/_pytest/hookspec.py	100% <0%> (ø)	⬆️
src/_pytest/fixtures.py	97.91% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.24% <0%> (+0.3%)	⬆️
src/_pytest/pytester.py	87.44% <0%> (+0.42%)	⬆️
src/_pytest/nodes.py	95.52% <0%> (+0.99%)	⬆️
... and 6 more

---

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[asottile]

heh -- will have to fixup pygments-pytest to know about this as well

(EDIT: it won't block the release like the last time though since it only displays in old pythons)