Pyvisa-py

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- fix usbtmc to use MaxPacketSize reported by endpoint PR 417

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- add URL-support to ASLR devices PR 386
- add support for GPIB secondary addresses
- fix missing sock.close() in rpc _connect()
- Adjusted how `iter_bytes` works to be more accurate to the VISA spec and removed
it from the `serial` module (it can still be found in `common`)
- fix HiSLIP message tracking after read timeout PR 376
- handle read_termination of null in tcipip PR 394
- fix tcpip keepalive PR 396
- store more attributes for USB resources PR 399

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- add support for the flush operation with TCPIP::SOCKET resources PR 350
- drop support for Python 3.7 PR 362
- fix listing of available resources PR 362
- fix hislip support for custom sub_addresses PR 359
- fix bad USBRaw resource preventing enumeration of other resources PR 370

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- fix bad behavior on PyVISA 1.12 and hence on Python 3.7 PR 357
0.6.x is the last version that will support Python 3.7

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- fix usb resource handling by avoiding multiple calls to set_configuration PR 352
- formatting fixes on files using "black" PR 352

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- fix listing resources when some optional dependencies are missing PR 349
- properly list discovered TCPIP resources PR 349
- fix pyvisa-info output for TCPIP and GPIB resources PR 349