Give a warning when using citation.get_systems() without Cython profiling

[jdemeyer]

sage.misc.citation.get_systems() is implemented by using cProfile to look at which modules implement the functions called when executing the code.

The problem is that this is totally unreliable when Cython is compiled without profiling support (which is the default). This doctest

sage: from sage.misc.citation import get_systems
sage: get_systems('((x+1)^2).expand()')
['ginac']

only works because Expression.expand() is called by Python instead of Cython. If that call would be inside some other Cython code, then Python's profiler would not detect it:

sage: cython('def callexpand(x): return x.expand()')
sage: from sage.misc.citation import get_systems
sage: get_systems('callexpand(((x+1)^2))')
[]

There is a problem because #22747 will "break" profiling even further as even the top-level call of Expression.expand() would not be detected as something to be entered in the profiler.

So here I propose simply to give a warning whenever get_systems() is used when profiling was not enabled.

CC: @seblabbe

Component: cython

Keywords: thursdaysbdx

Author: Jeroen Demeyer

Branch/Commit: ae120c5

Reviewer: Sébastien Labbé

Issue created by migration from https://trac.sagemath.org/ticket/23162

[jdemeyer]

Branch: u/jdemeyer/give_a_warning_when_using_citation_get_systems___without_cython_profiling

[jdemeyer]

Commit: ae120c5

[jdemeyer]

Author: Jeroen Demeyer

[jdemeyer]

New commits:

ae120c5

Give a warning when using citation.get_systems() without Cython profiling

[seblabbe]

comment:4

Works for me on OSX 10.10. sage -t --long works. Code looks good. Documentation builds and looks fine.

[seblabbe]

Reviewer: Sébastien Labbé

[seblabbe]

Changed keywords from none to thursdaysbdx

[vbraun]

Changed branch from u/jdemeyer/give_a_warning_when_using_citation_get_systems___without_cython_profiling to ae120c5