lint: ErrorWrapper.write: remove assertion

[blueyed]

With webpy non-bytes might arrive here, via:

print(traceback.format_exc(), file=web.debug)
out = ctx.environ['wsgi.errors']
out.write(x)

I think this is legitimate, and paste should not cause more trouble
(i.e. a new exception) in this case.

[codecov-io]

Codecov Report

Merging #18 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   53.52%   53.52%   -0.01%     
==========================================
  Files         125      125              
  Lines       14758    14757       -1     
  Branches     2501     2501              
==========================================
- Hits         7899     7898       -1     
- Misses       6307     6308       +1     
+ Partials      552      551       -1

Flag	Coverage Δ
#py27	52.38% <ø> (-0.05%)	⬇️
#py35	51.78% <ø> (-0.05%)	⬇️
#py36	51.78% <ø> (-0.05%)	⬇️
#py37	51.81% <ø> (-0.01%)	⬇️
#pypy	51.64% <ø> (-0.05%)	⬇️

Impacted Files	Coverage Δ
paste/lint.py	82.32% <ø> (-0.1%)	⬇️
paste/session.py	75% <0%> (ø)	⬆️

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[cdent]

Have a look at https://bitbucket.org/ianb/paste/commits/ab9e5d22ad48eb70db8ef6cbc726ae809c397118
which appears to have made a bad interpretation of pep 3333:

Search 'wsgi.errors' in https://www.python.org/dev/peps/pep-3333/

Instead of removing the assertion (thus removing any linting), consider putting a version of the old code back: assert type(s) is type("")