Pixar recently released USD 20.02 [1]. I think it's important for people
to be able to figure out which version of the USD library is used in
Blender.
[1] https://github.com/PixarAnimationStudios/USD/releases/tag/v20.02
This commit exposes the USD library information via `bpy.app.usd`, and
includes that info in the `system-info.txt` saved via Help → Save System
Info.
Reviewed by: brecht
Differential Revision: https://developer.blender.org/D6724