Improve pre-commit detection for changelog filenames

[asottile]

Resolves #3955

Before:

$ pre-commit  run changelogs-rst --all-files
changelog filenames......................................................Failed
hookid: changelogs-rst

changelog files must be named ####.(feature|bugfix|doc|removal|vendor|trivial).rst

changelog/3251.feture.rst

[asottile]

note here that language: fail is a new pre-commit "language"

[coveralls]

Coverage increased (+0.02%) to 93.902% when pulling 826adaf on asottile:changelog_files into 3dd2933 on pytest-dev:master.

[coveralls]

Coverage increased (+0.02%) to 93.902% when pulling 826adaf on asottile:changelog_files into 3dd2933 on pytest-dev:master.

[codecov]

Codecov Report

Merging #3957 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3957      +/-   ##
==========================================
+ Coverage   94.48%   94.52%   +0.04%     
==========================================
  Files         107      107              
  Lines       23651    23651              
  Branches     2349     2349              
==========================================
+ Hits        22347    22357      +10     
+ Misses        993      987       -6     
+ Partials      311      307       -4

Flag	Coverage Δ
#doctesting	28.53% <ø> (+0.06%)	⬆️
#linux	94.4% <ø> (+0.04%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.21% <ø> (+0.06%)	⬆️
#pexpect	0% <ø> (ø)	⬆️
#pluggymaster	93.63% <ø> (+0.03%)	⬆️
#py27	92.63% <ø> (+0.04%)	⬆️
#py34	92.18% <ø> (+0.03%)	⬆️
#py35	92.19% <ø> (+0.03%)	⬆️
#py36	92.76% <ø> (+0.04%)	⬆️
#py37	92.39% <ø> (+0.03%)	⬆️
#trial	31.29% <ø> (+0.06%)	⬆️
#windows	93.82% <ø> (ø)	⬆️
#xdist	18.48% <ø> (+0.02%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/terminal.py	91.6% <0%> (+1.74%)	⬆️

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