Unknown file type for field extraction: foo.kicad_sym

[mangelozzi]

Describe the bug The new KiCAD v6 .kicad_sym files types is not supported

To Reproduce Steps to reproduce the behavior:

1. Try kifield --extract foo.kicad_sym --insert bar.xlsx

Expected behavior To extract the fields

Desktop (please complete the following information):

• OS [type & version]: Windows 10 2004
• Python version 3.8.2
• KiField version 0..1.17 (current pip version)
• KiCad version 5.99

Actual behaviour

$ kifield --extract foo.kicad_sym --insert here.xlsx
Unknown file type for field extraction: foo.kicad_sym.
No part fields were extracted from these files: foo.kicad_sym
  * Did you provide a list of files?
  * Do these files exist?
  * Did you annotate your components in the schematic?
Traceback (most recent call last):
  File "/home/michael/venv/project/bin/kifield", line 11, in <module>
    load_entry_point('kifield==0.1.17', 'console_scripts', 'kifield')()
  File "/home/michael/venv/project/lib/python3.8/site-packages/kifield/__main__.py", line 159, in main
    kifield(
  File "/home/michael/venv/project/lib/python3.8/site-packages/kifield/kifield.py", line 1352, in kifield
    clean_part_fields(part_fields_dict)
  File "/home/michael/venv/project/lib/python3.8/site-packages/kifield/kifield.py", line 1330, in clean_part_fields
    for part, fields in part_fields_dict.items():
AttributeError: 'NoneType' object has no attribute 'items'
```