Wrong cache timing for long-running functions

[goncalopp]

I'm not sure if this actually a bug, or intended behaviour, but cached_property_with_ttl doesn't consider the time that the cached function takes to execute, only the time between attribute accesses.
This means that, for long-running functions, the cache misses when it shouldn't (my use case is caching the result of HTTP requests).

Here's code that demonstrates the issue

from cached_property import cached_property_with_ttl
import time

class C(object):
    @cached_property_with_ttl(ttl=1)
    def value(self):
        print("computing")
        time.sleep(2)

c = C()
c.value
c.value

The function is called twice, despite the fact that the second attribute access happens "immediately"

This can be solved with the following patch

            if not ttl_expired:
                return value

+      now = time()
        value = self.func(obj)
        obj_dict[name] = (value, now)
        return value

[ljluestc]

from cached_property import cached_property_with_ttl
import time

class C(object):
@cached_property_with_ttl(ttl=1)
def value(self):
print("computing")
time.sleep(2)

Add this import for the 'time' function

from time import time as current_time

c = C()
c.value
c.value