Remove dead UnixWare, SCO OpenServer and ReliantUNIX support code

[vasiliyk]

Feature or enhancement

Proposal:

CPython still carries build logic and a few C preprocessor statements for SCO UnixWare 7, OpenUNIX 8, SCO OpenServer 5 and ReliantUNIX, mostly in configure.ac.

This code seems dead and I propose removing it.

Rationale

• None of these platforms has ever been in a PEP 11 tier, and Python 3 does not claim support for them.
• The code has not been touched since 2003, and the last report from any of these systems is Compiling Python 2.7.10 Error on Unixware 7.1.4 #68799 (2015), a Python 2.7 build failure.
• The code targets compilers from around 2000 and predates the C11 requirement introduced in 3.11.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-157949: Remove dead UnixWare, OpenServer and ReliantUNIX support code #157950