s-tui: migrate to python3

Includes LC_ALL = "en_US.UTF-8"; that resolves a locale.Error exception
during the check phase.

[...]
  File "/build/s-tui-1.0.0/s_tui/sensors_menu.py", line 27, in <module>
    import urwid
  File "/nix/store/xxhpq1kcjy0kimfwnwqlzh2pchkp9khi-python3.7-urwid-2.1.0/lib/python3.7/site-packages/urwid/__init__.py", line 26, in <module>
    from urwid.widget import (FLOW, BOX, FIXED, LEFT, RIGHT, CENTER, TOP, MIDDLE,
  File "/nix/store/xxhpq1kcjy0kimfwnwqlzh2pchkp9khi-python3.7-urwid-2.1.0/lib/python3.7/site-packages/urwid/widget.py", line 27, in <module>
    from urwid.util import (MetaSuper, decompose_tagmarkup, calc_width,
  File "/nix/store/xxhpq1kcjy0kimfwnwqlzh2pchkp9khi-python3.7-urwid-2.1.0/lib/python3.7/site-packages/urwid/util.py", line 61, in <module>
    detected_encoding = detect_encoding()
  File "/nix/store/xxhpq1kcjy0kimfwnwqlzh2pchkp9khi-python3.7-urwid-2.1.0/lib/python3.7/site-packages/urwid/util.py", line 58, in detect_encoding
    locale.setlocale(locale.LC_ALL, initial)
  File "/nix/store/ja04f3cmapzb3f2mvjrb883bfqclsirq-python3-3.7.6/lib/python3.7/locale.py", line 608, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting