Deprecate click.get_terminal_size() in favor of stdlib shutil.

[jdufresne]

Since Python 3.3, the stdlib provides the function
shutil.get_terminal_size(). Now that the project has dropped Python 2
support, the compatibility shim is no longer necessary.

The stdlib version returns a named tuple. So rather than indexing "0",
can use the more self-documenting attribute "columns".

Docs available at:
https://docs.python.org/3/library/shutil.html#shutil.get_terminal_size

Fixes #1736

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.