Merge branch 'process_spider_exception_generator_experiment' into process_spider_exception_generator

update default RETRY_HTTP_CODES on docs

This reverts commit ba294351.

[MRG+1] Remove deprecated CrawlerSettings class and Settings attributes

[MRG+1] Update depth middleware stats (fixes #3245)

[MRG+1] add ftp to the scheme list

Python 3.7 support

Capture traceback when trying to import required twisted modules,
print it in case telnet is enabled, and mention settings variable
that can be used to supress the message.
Thanks @kmike!

Disable telnet console if it's not available, else we'll get an extra
warning about failure to enable it, and tests will fail.

See https://github.com/travis-ci/travis-ci/issues/9815#issuecomment-401756442
and https://github.com/travis-ci/travis-ci/issues/9815#issuecomment-402045581

PEP 479 does not allow for StopIteration in generators.  Instead,
handle it explicitly, also use a for loop which looks simpler.

Also give a more detailed reason why telnet is not enabled (for the
future).

2.9 gives collection errors on python 3.7 due to PEP 479.

[MRG+1] Delete relocation shims