Avoid python3 warning about not properly closing a file.

[marktucker]

Description of Change(s)

When running usdview in a python3 environment, a warning like the following could be output:

.....\python39\lib\site-packages\pxr\Usdviewq\appController.py:419: ResourceWarning: unclosed file <_io.TextIOWrapper name='...../python39/lib/site-packages/pxr/Usdviewq/usdviewstyle.q
ss' mode='r' encoding='cp65001'>
  self._setStyleSheetUsingState()

This change uses with open as to fix this problem.

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[jilliene]

Filed as internal issue #USD-7373