QGIS3 python 2 error

[fiftysevendegreesofrad]

Why does this happen - QGIS3 shouldn't be running python2?

Converting input layer to shapefile: C:\Users\User\AppData\Local\Temp\tmp173fhf8z.shp
Running external command: "C:\OSGeo4W64\bin\python3.exe" "c:\Program Files (x86)\sDNA\bin\sdnaprepare.py" --im "net=C:/Users/User/AppData/Local/Temp/tmp173fhf8z.shp" --om "net=C:/Users/User/AppData/Local/Temp/processing_ZwoLaM/63f337697535409f8901d0a1c9487a35/output.shp;errors=C:/Users/User/AppData/Local/Temp/processing_ZwoLaM/63f337697535409f8901d0a1c9487a35/output.shp" "start_gs=;end_gs=;xytol=;ztol=;action=DETECT;nearmisses;duplicates;isolated;island=;data_absolute=;data_unitlength=;data_text="
ERR: Fatal Python error: initfsencoding: unable to load the file system codec
ERR: File "C:\OSGeo4W64\apps\python27\Lib\encodings_init_.py", line 123
ERR: raise CodecRegistryError,
ERR: ^
ERR: SyntaxError: invalid syntax
ERR: Current thread 0x00003368 (most recent call first):
External command completed
Execution completed in 0.88 seconds
Results:
{'OUTPUT': <sdna.sdna_plugin_algorithm.ShapefileParameterVectorDestination object at 0x0000025392A1C0D8>}