Only print warning when using old cothread version

[AlexanderWells-diamond]

Everything from cothread 2.16+ is safe to use alongside epicscorelibs.

This has been manually tested with Python 2.7 with cothread 2.13.1 through 2.18.1, and Python 3.8 with cothread 2.16 through 2.19.1.

Fixes #22

CC @Araneidae

[mdavidsaver]

I would prefer to minimize the number of module level variables. So either del the new module variables, especially cothread, or move this check into a function.

Maybe something like:

def check_cothread_order():
    cothread = sys.modules.get("cothread")
    # >= 2.16 will attempt to import this module
    # >= 2.19 adds cothread._version
    ver = getattr(cothread, "_version", None) or tuple(int(c) for c in cothread.__version__.split("."))
    if ver < (2, 16):
        warnings.warn("epicscorelibs.path.cothread must be imported before cothread.catools to have effect")
check_cothread_order()
del check_cothread_order

[AlexanderWells-diamond]

Thank you @mdavidsaver.

There was a slight issue with the suggestion, as getattr(cothread, "_version", None) returns a Module object rather than the version string in newer Cothread versions. I figure it's simplest to just always find and parse the version string in all cases.