tests/sitecustomize.py

util: add the code path to "cachefunc()" for the function taking no arguments

Before this patch, "util.cachefunc()" caches the value returned by the
specified function into dictionary "cache", even if the specified
function takes no arguments.

In such case, "cache" has at most one entry, and distinction between
entries in "cache" is meaningless.

This patch adds the code path to "cachefunc()" for the function taking
no arguments for efficiency: to store only one cached value, using
list "cache" is a little faster than using dictionary "cache".

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass