Add --strip-extras option to pip-compile for producing constraint compatible output

[ssbarnea]

Changelog-friendly one-liner: Add an option for producing constraint-compatible output

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

Fixes: #1300

[webknjaz]

Nit: it's best to use a literal

Suggested change

	extras_require = dict()
	extras_require = {}

[anotinelg]

h @atugushev

I have noticed that when i use pip-compile to create a requirements.txt from a requirements.in for some package it adds sometimes an extra part on the line like

s3fs==0.4.2; extra == "myextra"
    # via mypackage

I try to use the pip-compile with the --strip-extras, but i still have this extra part.
Questions:

• In what case, does pip-compile add some extra
• Is the stip-extra parameter not supposed to remove all extras part?

my requirements.in looks like

-e .[myextra]

and the package myextra has a setup.py which specifies the extra like the following:

# -*- coding: utf-8 -*-
from setuptools import setup

# Declare minimal set for installation
required_packages = []

extras = {
}

extras["myextra"] = [
        "opencv_python == 4.1.2.30",
        "sklearn",
        "s3fs==0.4.2",
        "joblib",
        "mxnet==1.7.0.post1",
         "pandas", "findspark", "joblib"
    ]

setup(use_scm_version=True, install_requires=required_packages, extras_require=extras)

the command i used to create the requirements.txt is:

pip-compile requirements.in --output-file  requirements.txt

i am running pip-compile v6.3.0, and pip version v12.2.2.

many thanks for your help!!!