Add support for SDRPlay v3 API

Expected Behavior

The RSPdx is a great radio - very sensitive but... SDRplay has native support on URH, but the new RSPdx is not working correctly: Device seen as "RSP 3" Antenna A only is allowed (If I select B it reverts back to A) RF signals are MUCH weaker at the RF and IF gain settings than seen with SDRuno With the same settings RTL-SDR shows much stronger RF signals (Realtek RTL2838UHIDIR)

Actual Behavior

(see above)

Steps To Reproduce

Plug SDRdx in and run Spectrum Analyzer

Screenshots

Platform Specifications

• OS: Windows 10, 64 bit
• URH version: 2.8.8
• Python version: (not applicable: Python 3.8.3)
• Installed via Windows installer