Depend on fasteners >= 0.16 to remove obselete dependency on monotonic

[Zac-HD]

gsutil declares a dependency on the obsolete monotonic package, which was a substitute for the stdlib time.monotonic() function before that was available on all platforms (from Python 3.5 onwards).

However, this is only necessary for the very old vendored copy of fasteners (at v0.14), which at version 0.16 dropped this dependency. In my environment, the monotonic package is never used at all: gsutil doesn't use it directly, and the version of fasteners I'm using doesn't use it either.

I'd therefore encourage you to drop this un-needed and obsolete dependency, either by updating the vendored copy of fasteners, or at least moving the dependency from gsutil into your vendored library so that it doesn't get pulled in from PyPI for other users. This may seem like a small thing but image size and auditing times add up surprisingly quickly!