python: Errors regarding to the missing `--single-version-externally-managed` option

[林博仁(Buo-ren, Lin)]

The following error occurred when I tried to build a python plugin recipe:

```
/root/parts/picard/install/usr/bin/python3 setup.py --no-user-cfg install --single-version-externally-managed --user --record install.txt
usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: setup.py --help [cmd1 cmd2 ...]
   or: setup.py --help-commands
   or: setup.py cmd --help

error: option --single-version-externally-managed not recognized
```

Maybe related: https://stackoverflow.com/questions/14296531/what-does-error-option-single-version-externally-managed-not-recognized-ind

Does this option really matter in snap packaging? Currently I use a custom plugin that simply dropped the offending option.