Remove workaround for docstrings for py38+

[asottile]

test-only changes for #4716

$ pytest testing/test_assertrewrite.py 
============================ test session starts ============================
platform darwin -- Python 3.8.0a1+, pytest-4.2.1.dev15+ga9457345, py-1.7.0, pluggy-0.8.1
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/Users/asottile/workspace/cpython/pytest/.hypothesis/examples')
rootdir: /Users/asottile/workspace/cpython/pytest, inifile: tox.ini
plugins: hypothesis-4.5.1
collected 66 items                                                          

testing/test_assertrewrite.py ....................................... [ 59%]
....s..ssss................                                           [100%]
========================== short test summary info ==========================
SKIPPED [1] testing/test_assertrewrite.py:866: python 2 only
SKIPPED [1] testing/test_assertrewrite.py:915: condition: sys.version_info[0] >= 3
SKIPPED [1] testing/test_assertrewrite.py:924: condition: sys.version_info[0] >= 3
SKIPPED [1] testing/test_assertrewrite.py:935: condition: sys.version_info[0] >= 3
SKIPPED [1] testing/test_assertrewrite.py:946: condition: sys.version_info[0] >= 3

=================== 61 passed, 5 skipped in 6.76 seconds ====================

[codecov]

Codecov Report

Merging #4723 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4723      +/-   ##
==========================================
+ Coverage   95.68%   95.68%   +<.01%     
==========================================
  Files         113      113              
  Lines       24970    24963       -7     
  Branches     2479     2478       -1     
==========================================
- Hits        23892    23886       -6     
  Misses        762      762              
+ Partials      316      315       -1

Flag	Coverage Δ
#docs	29.67% <0%> (+0.08%)	⬆️
#doctesting	29.67% <0%> (+0.08%)	⬆️
#linting	29.67% <0%> (+0.08%)	⬆️
#linux	95.5% <100%> (ø)	⬆️
#nobyte	92.29% <100%> (ø)	⬆️
#numpy	93.1% <100%> (-0.01%)	⬇️
#pexpect	42.1% <0%> (+0.01%)	⬆️
#py27	93.69% <100%> (ø)	⬆️
#py34	91.77% <100%> (+0.06%)	⬆️
#py35	91.77% <100%> (+0.06%)	⬆️
#py36	91.79% <100%> (+0.06%)	⬆️
#py37	93.8% <100%> (+0.02%)	⬆️
#trial	93.1% <100%> (-0.01%)	⬇️
#windows	93.87% <100%> (-0.01%)	⬇️
#xdist	93.71% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
testing/test_assertrewrite.py	83.48% <100%> (-0.03%)	⬇️

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