Pytube

* Now correctly raises non-timeout URLErrors instead of MaxRetriesExceeded

* Defaults to a duration of 0.0s when 'dur' key is missing from XML subtitles.

* Consolidates some import lines to reduce clutter at top of files.

* Adds network timeout and retries to stream.download() method

* Removes the typing_extensions install requirement from setup.py, which was missed when it was previously "removed".

* Defers web requests until they're actually necessary to fetch information

* Adjusts tests to reflect changes to how web requests are made

* Removes "defer_prefetch_init" arg from YouTube object initialization

* Converted most attributes into caching properties to facilitate request deferring.

* Added some documentation to uniqueify to explain its purpose.

* Prefetching added to conftest to improve test speed

* Added some setters for YouTube properties.

* Some cosmetic README changes

* Fix for the new youtube playlist requests

* Implements pytube.request.post in order to fetch continuations

* Implements ytcfg extraction

* Slight refactor of object parser

* Extends request_execute_request to take a data argument for post requests

* Adjusted tests to reflect changes