Use pprint.pformat on overridden settings

[grammy-jiang]

This commit use pprint.pformat on overridden settings to keep consistent
with scrapy.middleware, for example:

Original:

2019-11-28 22:23:57 [scrapy.crawler] INFO: Overridden settings: {'BOT_NAME': 'spider_demo', 'NEWSPIDER_MODULE': 'spider_demo.spiders', 'SPIDER_MODULES': ['spider_demo.spiders'], 'USER_AGENT': 'Mozilla/5.0 (X11; Fedora; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.157 Safari/537.36'}
2019-11-28 22:23:57 [scrapy.middleware] INFO: Enabled extensions:
['scrapy.extensions.corestats.CoreStats',
 'scrapy.extensions.telnet.TelnetConsole',
 'scrapy.extensions.memusage.MemoryUsage',
 'scrapy.extensions.logstats.LogStats']

2019-11-28 22:23:57 [scrapy.crawler] INFO: Overridden settings:
{'BOT_NAME': 'spider_demo',
 'NEWSPIDER_MODULE': 'spider_demo.spiders',
 'SPIDER_MODULES': ['spider_demo.spiders'],
 'USER_AGENT': 'Mozilla/5.0 (X11; Fedora; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.157 Safari/537.36'}
2019-11-28 22:23:57 [scrapy.middleware] INFO: Enabled extensions:
['scrapy.extensions.corestats.CoreStats',
 'scrapy.extensions.telnet.TelnetConsole',
 'scrapy.extensions.memusage.MemoryUsage',
 'scrapy.extensions.logstats.LogStats']

[codecov]

Codecov Report

Merging #4199 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4199      +/-   ##
==========================================
- Coverage   83.74%   83.73%   -0.01%     
==========================================
  Files         165      165              
  Lines        9713     9714       +1     
  Branches     1446     1446              
==========================================
  Hits         8134     8134              
  Misses       1332     1332              
- Partials      247      248       +1

Impacted Files	Coverage Δ
scrapy/crawler.py	90% <100%> (+0.05%)	⬆️
scrapy/utils/trackref.py	83.78% <0%> (-2.71%)	⬇️

[elacuesta]

If I'm not mistaken this is technically a slightly backward incompatible change - any log parsers which handle job's text output would need to adapt to this new format. I'm in favor of the patch, but we might want to mention this fact somewhere in the release notes.