Spelling mistake

Fix DNS resolver when DNSCACHE_ENABLED=False

Fix PyPy test failures

Thanks for the catch @redapple

LevelDB does not have "official" close method, so we have
to rely on garbage collection to close it.

Using https://github.com/lopuhin/pydispatcher, pypy branch.
This is executed as a separate step to avoid changing
default requirements.txt and setup.py. If just added to "deps"
in tox, this install command will be executed as one command
and PyPyDispatcher will not override PyDispatcher.

On CPython get_func_args does not work correctly for built-in
methods.

One gc.collect() seems to be enough, but it's more reliable
to run it several times (at most 100), until all objects are collected.

It is not affected by Twisted bug #7989 and is more permissive
with pickling (especially with protocol=2).

For float(u'$10') PyPy includes "u'" in the error message,
and it's more fair to check error message on input we are really
passing.

Twisted prints errors in DebugInfo.__del__, but PyPy does not run
gc.collect() on exit:
http://doc.pypy.org/en/latest/cpython_differences.html?highlight=gc.collect#differences-related-to-garbage-collection-strategies

use suggest method instead of DEPRECATED one

Update extensions.rst

#2759

Include references to Scrapy subreddit in the docs

Added missing bullet point for the AUTOTHROTTLE_TARGET_CONCURRENCY setting.

Release notes for 1.4.0