Added python 2 deprecation notice

[danimtb]

• Refer to the issue that supports this Pull Request: closes Print Warning for Python 2 in conan client #3531
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one. Also adding a description of the changes in the changelog.rst file. https://github.com/conan-io/docs

Docs already have a python 2 deprecation notice

Changelog: Feature: Added python 2 deprecation message in the output of the conan commands.

[memsharded]

Better keep it minimal at the moment, 1-2 warning lines in yellow should be enough. Tests broken because of extra output, thinking about it, it might affect some commands that people might be parsing the output (conan remote list, conan info --build-order, etc), but I don't think we have to commit on not changing the conan output in that case (cc/ @lasote)

[danimtb]

Fine, will keep it short

[danimtb]

Finally the message does NOT appear in commands with high probabilities of being parsed:

• conan info
• conan get
• conan remote
• conan config
• conan user
• conan search
• conan profile

If you think there is any other that should not show the message just point it out here.