[issue42789] Do not skip test_curses on non-tty

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

s390x RHEL8 3.x:
https://buildbot.python.org/all/#/builders/509/builds/601

Tests run with TERM=xterm-256color.

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ERROR: test_colors_funcs (test.test_curses.TestCurses)
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Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z/build/Lib/test/test_curses.py", line 316, in test_colors_funcs
    curses.pair_content(curses.COLOR_PAIRS - 1)
_curses.error: Argument 1 was out of range. (1..COLOR_PAIRS-1)

--

test.pythoninfo doesn't log anything about curses. I expected to find _curses.ncurses_version there.

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