Libusb

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- Ability to specify the underlying shared library programmatically.
- General update and cleanup.
- Setup update.
- Removing dependence on atpublic.
- Fixed docs setup.
- Fix for hotplugtest example.

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- | Upgrade for Windows: libusb v.1.0.22 -> v.1.0.23
| (partially; without libusb_wrap_sys_device, because
| original Windows v.1.0.23 dlls do not export this function).
- Add support for Python 3.9
- Drop support for Python 3.5
- Setup update.
- Cleanup.

* New libusb_set_option() API
* Fix transfer timeout not being cleared upon resubmission
* Report super speed plus devices on modern Linux and macOS
* Darwin: Improve support for macOS Sierra and High Sierra
* Darwin: SDK fixes and improvements
* Linux: Let initialization succeed when no devices are present
* Linux: Mark internal file descriptors with CLOEXEC flag
* Solaris: Add support for attach/detach kernel driver
* Windows: Add dynamic UsbDk backend selection
* Windows: Add isochronous transfer support via libusbK
* Windows: Add Visual Studio 2017 support
* Windows: Fix enumeration problems on Windows 8 and later
* Windows: Major rework of poll() emulation
* Windows: Numerous HID API fixes
* Windows: Support cancellation of individual transfers (Vista and later)
* Various other bug fixes and improvements

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- Added ReadTheDocs config file.
- Setup update.

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- Upgrade for Linux: libusb x64 v.1.0.21 -> v.1.0.22
- Fix for error when the shared library is configured.
- Cleanup.

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- Initial support for Linux (libusb v.1.0.21 x64).