Add more dependencies to extras_require (eg sqlparse, PyYAML)

[Ed Morley]

There are several optional Django features that require third-party Python packages that (intentionally) aren't specified as install_requires dependencies in setup.py, since not everyone would want them installed.

For example:

• PyYAML for the yaml serializer
• sqlparse for multi-line SQL statements for migrations RunSQL
• Database drivers

Adding these as extras_require [*] entries has the following advantages:

1) The reason for the packages is now self-documenting. eg:

Django[sqlparse,yaml]

...instead of:

Django

# Required by Django's migration system for RunSQL commands.
sqlparse

# Required by Django's YAML serializer.
PyYAML

2) Django could in the future manage the preferred package for a particular feature in cases where there is a choice (eg mysqlclient vs MySQL-python). Users can still override by not using the extras_require alias (or there could even be multiple aliases).

3) If a new major version of Django stops requiring a particular extras_require dependency, users will know to remove it from their requirements files, since pip will warn there is no such extra listed.

[*] ​http://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

[Josh Smeaton]

I think I like this idea, but it's probably a change sufficient enough to bring up on the mailing list ​https://groups.google.com/forum/#!forum/django-developers

[Carlton Gibson]

I'm going to resolve this as a duplicate of #28905. Whilst this issue was prior, #28905 is further along. (It has a PR)