fix(drivers): run on BSD and other unix

[slook]

• Fixed: ImportError when running unittests on BSD and derivatives

Example CI runner output on OpenBSD (same exception was in my runners for FreeBSD, Solaris, etc):

test_tui_startup (pynicotine.tests.integration.test_startup.StartupTest.test_tui_startup)
Verify that TermTk TUI startup works. ... Using TTk executable: /home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk
OpenBSD
Unsupported system.
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/__main__.py", line 7, in <module>
    sys.exit(run())
             ^^^^^
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/__init__.py", line 224, in run
    exit_code = application.run(ci_mode, isolated_mode)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/ttktui/__init__.py", line 29, in run
    error = check_ttk_version()
            ^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/ttktui/__init__.py", line 11, in check_ttk_version
    from TermTk import __version__ as TTk_version
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/__init__.py", line 25, in <module>
    from .TTkTheme       import *
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkTheme/__init__.py", line 1, in <module>
    from .theme import *
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkTheme/theme.py", line 25, in <module>
    from TermTk.TTkCore.color import TTkColor
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/__init__.py", line 6, in <module>
    from .helper   import *
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/helper.py", line 30, in <module>
    from TermTk.TTkCore.TTkTerm.term import TTkTerm
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/TTkTerm/__init__.py", line 4, in <module>
    from .input      import *
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/TTkTerm/input.py", line 26, in <module>
    from .input_thread import TTkInput
  File "/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/TTkTerm/input_thread.py", line 32, in <module>
    from ..drivers import TTkInputDriver
ImportError: cannot import name 'TTkInputDriver' from 'TermTk.TTkCore.drivers' (/home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/drivers/__init__.py)
FAIL

After fix in this PR (my project's CI runner is currently only able to test that the library is successfully imported):

Verify that TermTk TUI startup works. ... Using TTk executable: /home/runner/work/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk
OpenBSD
Imported TermTk 0.49.0-a0
Loaded TTk 0.49.0-a0
Nicotine+ TTk loaded okay, but not a TTY!
ok

Note this fix assumes that 'OpenBSD', 'FreeBSD', 'NetBSD', 'Haiku', 'SunOS' are all like 'Linux', but those operating systems cannot actually be tested at runtime by anyone currently because this ImportError makes running on those platforms impossible for no good reason (the Linux driver should work on any platform that has the standard termios package).

If any issues are reported then alternate terminal driver modules can be imported for different platforms if required.