TST exclude lxml==4.2.2 from tests

[kmike]

This PR is an example of Travis-driven development, as I can't reproduce an issue locally on OS X.

1. forcing image to loading before temp file is gone - not a fix.
2. reverting back to BytesIO, as a Pillow issue seems to be solved (see #803 (comment) for motivation for moving to TemporaryFile) - not a fix, though probably it makes the code cleaner.

TODO: fix it. It seems the issue is related to PNG codec, as JPEG images work.

[codecov]

Codecov Report

Merging #3308 into master will increase coverage by 3.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3308      +/-   ##
==========================================
+ Coverage   78.72%   81.74%   +3.01%     
==========================================
  Files         228      228              
  Lines        9605     9605              
  Branches     1337     1337              
==========================================
+ Hits         7562     7852     +290     
+ Misses       1773     1487     -286     
+ Partials      270      266       -4

Impacted Files	Coverage Δ
scrapy/core/scraper.py	86.39% <0%> (+1.36%)	⬆️
scrapy/pipelines/files.py	67.51% <0%> (+1.45%)	⬆️
scrapy/http/headers.py	98.43% <0%> (+1.56%)	⬆️
scrapy/core/downloader/handlers/http11.py	92.5% <0%> (+1.87%)	⬆️
scrapy/utils/boto.py	73.33% <0%> (+6.66%)	⬆️
scrapy/core/downloader/tls.py	90% <0%> (+17.5%)	⬆️
scrapy/core/downloader/handlers/s3.py	95.16% <0%> (+22.58%)	⬆️
scrapy/downloadermiddlewares/httpcache.py	93.05% <0%> (+66.66%)	⬆️
scrapy/extensions/httpcache.py	94.82% <0%> (+71.72%)	⬆️

[grammy-jiang]

I have met this problem these days, and I have several PRs and their testings to wait to pass.

But sorry I am not familiar with the image related libraries, could not provide any help.

Hope this issue could be fixed soon.

[kmike]

@grammy-jiang this is actually caused by lxml (see python-pillow/Pillow#3208); lxml reverted the offending commit in a new release (lxml/lxml#265). So the fix would be to exclude bad lxml version, I'll do it in this PR.