app-accessibility/brltty: [QA] Fix multiple issues

app-accessibility/brltty: [QA] Fix multiple issues

* Don't add dead python impls to PYTHON_COMPAT
* Don't remove python metadata vars (${PYTHON_REQUIRED_USE})
  from REQUIRED_USE
* Don't remove the SLOT specifier for sys-libs/ncurses
* Get rid of the awfully complicated, hacky and
  non-generalisable solution for the libtinfo split,
  which also doesn't generalise to Prefix.
  Using pkg-config is easier, safer, more portable and
  less brittle than the previous hacks.
* Get rid of the totally unnecessary code paths
  created by dispatching on glibc versions.
  'sys/sysmacros.h' can always be included on glibc,
  packages just have relied on this include implicitly.

Package-Manager: Portage-2.3.16, Repoman-2.3.6