Usb attr fixes

[MatthieuDartiailh]

Closes #398

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a922159) 24.22% compared to head (32af7ae) 24.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #399   +/-   ##
=======================================
  Coverage   24.22%   24.22%           
=======================================
  Files          24       24           
  Lines        3356     3356           
  Branches      469      469           
=======================================
  Hits          813      813           
- Misses       2524     2525    +1     
+ Partials       19       18    -1     

Flag	Coverage Δ
unittests	24.22% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
pyvisa_py/usb.py	11.62% <0.00%> (ø)

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.