Allow debugger interactions in tests

[kdeldycke]

This PR patches CliRunner to allow pdb interactions in tests.

CliRunner captures stdout, which breaks pdb.set_trace() and interactive debuggers. You can't debug inside an invoked command because stdout is redirected.

By playing nice with pdb in Click tests, we can improve the experience of developers and maintainers, and as a far reaching goal, maybe improve bug reporting from the community.

This PR:

• addresses click.testing is not properly proxying stdout #654
• addresses click.testing.CliRunner.invoke prevents use of pdb #843
• addresses a side comment of ValueError: I/O operation on closed file in testing #824 (comment)
• supersedes an attempt at fixing it at WIP: testing: runner: wrap stdin+stderr #951