Re-arrange the imports in the httpproxy module

[grammy-jiang]

This commit re-arranges the imports in the httpproxy module to follow
pep8

[codecov]

Codecov Report

Merging #4210 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4210      +/-   ##
==========================================
+ Coverage   83.73%   83.74%   +0.01%     
==========================================
  Files         165      165              
  Lines        9714     9714              
  Branches     1446     1446              
==========================================
+ Hits         8134     8135       +1     
  Misses       1332     1332              
+ Partials      248      247       -1

Impacted Files	Coverage Δ
scrapy/downloadermiddlewares/httpproxy.py	100% <100%> (ø)	⬆️
scrapy/robotstxt.py	97.56% <0%> (ø)	⬆️
scrapy/extensions/memdebug.py	50% <0%> (ø)	⬆️
scrapy/extensions/debug.py	46.15% <0%> (ø)	⬆️
scrapy/extensions/throttle.py	46% <0%> (ø)	⬆️
scrapy/pipelines/__init__.py	92.3% <0%> (ø)	⬆️
scrapy/extensions/telnet.py	81.53% <0%> (ø)	⬆️
scrapy/extensions/memusage.py	48.91% <0%> (ø)	⬆️
scrapy/downloadermiddlewares/downloadtimeout.py	92.85% <0%> (ø)	⬆️
scrapy/utils/python.py	80.21% <0%> (ø)	⬆️
... and 58 more