Fix enum flag from cpp conversion

[skallweitNV]

• Use __str__ from the underlying flag factory class (allows printing flags like Flag.A|B|C)
• Fix conversion from cpp -> python of enum flags that contain a combination of flags.

[skallweitNV]

Hmm, looks like __str__ has changed in Python >= 3.11. It now returns numeric values for the Int variants.

[wjakob]

Could you add a testcase that checks the behavior of the flag __str__ routine?

[skallweitNV]

@wjakob this should be good to merge now.

[wjakob]

Thank you @skallweitNV!