Messages about ignoring packages due to environment markers should not be on stderr

[edmorley]

• Pip version: 9.0.1
• Python version: 2.7.14
• Operating system: Ubuntu 16.04

Description:

In one or our projects, we've added environment marker specifiers to our requirements file, such that the same file can be used with both Python 2 and 3 (as part of our gradual transition to Python 3).

Unfortunately the log output about skipping this package is output to stderr rather than stdout, which causes tools like Vagrant to output it red text compared to the green text of stdout. This confuses contributors to our project, since they think the provision didn't complete successfully, when in fact these messages are entirely expected and not an error.

IMO this output should be on stdout instead, by using logger.info() not logger.warning(), here:

pip/src/pip/_internal/req/req_set.py

Line 69 in b9feb19

logger.warning("Ignoring %s: markers '%s' don't match your "

What I've run:

1. Create a requirements file containing:

requests==2.18.4
aiohttp==2.3.2; python_version >= '3'

2. Using Python 2, run pip install on that requirements file

Expected:

There is nothing output on stderr, and stdout includes:
Ignoring aiohttp: markers 'python_version >= "3"' don't match your environment

Actual:

stderr includes:
Ignoring aiohttp: markers 'python_version >= "3"' don't match your environment

Example of the end result when being run via tooling that colours console output:

[edmorley]

This would also avoid issues like #4826.

[lock]

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