fix threading login in page getting
debug log for HTTPError
removed extraneous call to logger
block on queue.get and reuse worker threads
remove unused thread lock obj
just use a fixed 10 worker threads
update docstring
Move the thread queue management and variables from method to instance

Since the pool of workers gets reused, this seems the cleanest way
to ensure that excess threads aren't created as they no never return

because the queue is only ever used for one req at a time, it seems safe
to reuse it on the PackageFinder instance as _get_pages is called through:

prepare_files - find_requirement - _get_pages
fix thread args - remove unused queue size call
move ALL the thread getting state onto self

no sharing of seen and req variables
stub of thread helper class
better threading support for PackageFinder

adds a helper class that manages a threads and queue for page/link scraping
removed duplicate class stub

another glitch of the cherry-pick merge
verbose tests
add a reraise backcompat py2/3 func from six
handle exceptions in page getting threads
raise worker exception in main thread immediately
reuse the page_getter

we were creating 10 new threads per request
clarify exception surfacing from thread
test to check for threaded behavior