devel/py-stevedor: Add missing runtime dependency

stevedor uses pbr, which declares its build time (setup_requires) dependency
on it in setup.py. This is used to bootstrap pbr, which the package then uses to
declare other dependencies in setup.cfg or requirements.txt files.

stevedore declares the following in its requirements.txt file:

pbr!=2.1.0,>=2.0.0 # Apache-2.0
six>=1.10.0 # MIT

These are install_requires (RUN_DEPENDS), normally declared in setup.py files
for most python packages, notably except for those that use pbr.

ports r465381 inadvertently removed pbr from RUN_DEPENDS because it wasn't
listed in setup.py, which causes all consumers of stevedore to fail at runtime,
including multimedia/py-subliminal [1], with the following error:

pkg_resources.DistributionNotFound: The 'pbr!=2.1.0,>=2.0.0' distribution was not found stevedore

This has happened before [2].

Accordingly, this change re-adds pbr as a RUN_DEPENDS, and adds a comment
explaining that it's required at runtime, with references to previous
regressions to avoid footshooting in the future.

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234529
[2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197373

PR: 234529
Reported by: Luís Carneiro <luiscarneiro1993 yandex com>
Approved by: portmgr (blanket: run time, dependency fix, jfi)
MFH: 2018Q4