Release 1.5.1

extractred --> extracted

Add Python3 tools description.

Corrected minor grammatical issues and increased clarity of instructions.

I changed URLs to :class:`~scrapy.http.Request` in start_urls explanation of the default spider

Rectified grammatical errors

DOC typo fix in defer_fail docstring

Release notes for the upcoming 1.5.0 version 

Add mention to dont_merge_cookies in CookiesMiddlewares docs (#2999)

[MRG+1] Use HTTP pool and proper endpoint key for ProxyAgent

Add PyPy3 support

These built-in functions are exposed as methods in PyPy3.
For scrapy this does not matter as:
1) they do not work for CPython at all
2) get_func_args is checked for presense of an argument in scrapy,
   extra "self" does not matter.
But it still makes sense to leave these tests so that we know we
shouldn't use get_func_args for built-in functions/methods.

[MRG+1] setup.py: mention that we support PyPy. See GH-2213.