Rewrite "python" command exception handling

Rewrite "python" command exception handling

The "python" command (and the Python implementation of the gdb
"source" command) does not handle Python exceptions in the same way as
other gdb-facing Python code.  In particular, exceptions are turned
into a generic error rather than being routed through
gdbpy_handle_exception, which takes care of converting to 'quit' as
appropriate.

I think this was done this way because PyRun_SimpleFile and friends do
not propagate the Python exception -- they simply indicate that one
occurred.

This patch reimplements these functions to respect the general gdb
convention here.  As a bonus, some Windows-specific code can be
removed, as can the _execute_file function.

The bulk of this change is tweaking the test suite to match the new
way that exceptions are displayed.  These changes are largely
uninteresting.  However, it's worth pointing out the py-error.exp
change.  Here, the failure changes because the test changes the host
charset to something that isn't supported by Python.  This then
results in a weird error in the new setup.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31354
Acked-By: Tom de Vries <tdevries@suse.de>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>