Update redirect link from Python 2 docs to Python 3 docs

[adityaa30]

Description

The documentation has many redirect links to https://docs.python.org/2/*. As Scrapy now supports Python 3.5+ I think they should be changed to https://docs.python.org/3/* respectively.

Steps to Reproduce

1. Go to JsonItemExported and click on JSONEncoder
2. You will be redirected to Python 2 docs.

Above steps can be repeated for many other pages in Scrapy docs.

Expected behavior: Redirected to latest Python 3 docs

Actual behavior: Redirected to Python 2 docs

Reproduces how often: 100%

Versions

INFO:scrapy.utils.log:Scrapy 2.0.0 started (bot: scrapybot)
INFO:scrapy.utils.log:Versions: lxml 4.4.1.0, libxml2 2.9.9, cssselect 1.1.0, parsel 1.5.2, w3lib 1.21.0, Twisted 19.10.0, Python 3.6.10 (default, Feb 11 2020, 21:11:31) - [GCC 7.4.0], pyOpenSSL 19.1.0 (OpenSSL 1.1.1d 10 Sep 2019), cryptography 2.8, Platform Linux-5.3.0-40-generic-x86_64-with-debian-buster-sid
DEBUG:scrapy.utils.log:Using reactor: twisted.internet.epollreactor.EPollReactor
Scrapy : 2.0.0
lxml : 4.4.1.0
libxml2 : 2.9.9
cssselect : 1.1.0
parsel : 1.5.2
w3lib : 1.21.0
Twisted : 19.10.0
Python : 3.6.10 (default, Feb 11 2020, 21:11:31) - [GCC 7.4.0]
pyOpenSSL : 19.1.0 (OpenSSL 1.1.1d 10 Sep 2019)
cryptography : 2.8
Platform : Linux-5.3.0-40-generic-x86_64-with-debian-buster-sid

Additional context

I am willing to fix this issue, waiting for an approval.

[elacuesta]

You're right, thanks for pointing it out. Searching for "python.org/2" within the docs directory I could find many other occurrences of direct links. We are using Intersphinx now, and the stdlib is included in the mapping, probably most of those links could be rewritten. For instance, the JSONEncoder example would be:

:class:`json.JSONEncoder`

[adityaa30]

Thanks @elacuesta ! I will start working on this now.

[adityaa30]

@elacuesta I have sent a pr #4445 to fix this issue.