Allow estimate-difficulty to be exported (in Linux/macOS/Windows) as
independent executable, so that it can be used without having to install
Frama-C.

There are still many missing specifications, but at least parsing should
succeed for strictly POSIX-compliant code.

Some scripts use features which cannot or should not be tested
(e.g. reliance on an optional external tool); defining PTESTS_TESTING=1
allows these scripts to detect such circumstances and change (slightly)
their behavior to avoid non-reproducible oracles.

Remove obsolete 'minimal Python version' guards, which are likely insufficient,
due to the difficulty in testing them automatically. A better solution would
be useful.