Late-import base36 and QR code libraries; remove SUPPORT_QR_CODE flag

[akx]

Some tests run locally for home-assistant/core would fail with a cryptic "base36 is not defined" error if that library was not installed.

This PR:

• makes that error message cleaner – it's now a RuntimeError that has the import error as a reason
• makes pyhap not import base36 or QRCode at all until they are needed
  • in doing that removes the SUPPORT_QR_CODE flag; based on GitHub Code Search there are no external users for that flag that I can see.