Fix terminal size in tox commands (#2999)

[ziima]

Found it! And because it took me 6 hours to debug it, I'm going to describe how the debugging went.

At first, I started debugging the tox itself. Quickly I managed to discover, the issue can be triggered by python3 -c "from prompt_toolkit import prompt; prompt('Give me some input: ')" which behaved differently in tox and outside. After couple of hours trying to navigate the tox internals I landed at ReadViaThreadUnix where I noticed it print a significant number of '\r\n' pairs after the prompt. That led me to the prompt_toolkit. I spend some time there, until I discovered the screen it renders within tox has an unusual height. A but later I noticed the screen it tries to render have switched values of columns and rows when compared to COLUMNS and ROWS pass in env values by tox. Bit more digging lead to os.get_terminal_size(). The MWE was now just simply:

[testenv]
deps = ipython
commands =
    python3 -c "import os; print(os.get_terminal_size())"

# tox
py: commands[0]> python3 -c 'import os; print(os.get_terminal_size())'
os.terminal_size(columns=54, lines=189)

# .tox/py/bin/python3 -c 'import os; print(os.get_terminal_size())'
os.terminal_size(columns=189, lines=54)

I had no idea what to do now, but I didn't want to give up. I realized I could just grep tox source code for columns and I finally found match in _pty function. I tried to switch the two arguments and voila, works like a charm.

This PR is not yet finished, I haven't found how to design a test for this and I'm too tired at the moment. I hope to add it soon.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[ziima]

PR is ready.