Require funcsigs>=1.0 on Python 2.7

[nicoddemus]

Fix #4815

[codecov]

Codecov Report

Merging #4822 into features will decrease coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           features    #4822     +/-   ##
===========================================
- Coverage     95.75%   95.66%   -0.1%     
===========================================
  Files           113      113             
  Lines         25666    25163    -503     
  Branches       2505     2498      -7     
===========================================
- Hits          24577    24071    -506     
- Misses          770      772      +2     
- Partials        319      320      +1

Flag	Coverage Δ
#docs	29.59% <ø> (+0.1%)	⬆️
#doctesting	29.59% <ø> (+0.1%)	⬆️
#linting	29.59% <ø> (+0.1%)	⬆️
#linux	95.49% <ø> (-0.11%)	⬇️
#nobyte	92.2% <ø> (ø)	⬆️
#numpy	93% <ø> (ø)	⬆️
#pexpect	41.98% <ø> (ø)	⬆️
#pluggymaster	93.72% <ø> (+0.07%)	⬆️
#py27	93.68% <ø> (+0.03%)	⬆️
#py34	91.59% <ø> (ø)	⬆️
#py35	90.98% <ø> (-0.64%)	⬇️
#py36	91.67% <ø> (+0.03%)	⬆️
#py37	93.82% <ø> (ø)	⬆️
#py38	?
#trial	93% <ø> (ø)	⬆️
#windows	93.69% <ø> (+0.01%)	⬆️
#xdist	93.7% <ø> (-0.15%)	⬇️

Impacted Files	Coverage Δ
testing/test_modimport.py	81.81% <0%> (-1.52%)	⬇️
testing/code/test_source.py	95.53% <0%> (-1.07%)	⬇️
testing/test_argcomplete.py	68.11% <0%> (-0.9%)	⬇️
testing/test_compat.py	91.13% <0%> (-0.63%)	⬇️
testing/test_skipping.py	97.65% <0%> (-0.57%)	⬇️
src/_pytest/warnings.py	88.88% <0%> (-0.53%)	⬇️
src/_pytest/debugging.py	77.98% <0%> (-0.41%)	⬇️
src/_pytest/fixtures.py	97.91% <0%> (-0.31%)	⬇️
src/_pytest/logging.py	94.96% <0%> (-0.31%)	⬇️
testing/test_assertrewrite.py	83.55% <0%> (-0.3%)	⬇️
... and 73 more

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