Currently the Windows and macOS builds are done with PyQt 5.10 and PyQt 5.11 is blacklisted. This was done because of crashes of the Windows build when using PyQt 5.11.
However, there is no inherent incompatibility between the Picard source code and PyQt 5.11 and indeed Picard runs fine with PyQt 5.11 on Linux. The issues are rather some packaging and/or binary incompatibility.
Fix any issues and allow building with PyQt 5.11 on all platforms.