Update pydantic dependency to >= 2

[dvzrv]

I have used bump-pydantic on the code base to do the initial migration.
Afterwards I had to work on some details (e.g. __fields__ -> model_fields).

Please note, that I am very unsure about the changes to _get_pydantic_model_metadata(). Please review (or adapt as necessary)!

Fixes #72

[codecov]

Codecov Report

Merging #76 (5e10ea6) into master (6aca919) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 5e10ea6 differs from pull request most recent head 5a7f4e8. Consider uploading reports for the commit 5a7f4e8 to get more accurate results

@@            Coverage Diff            @@
##            master       #76   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          306       307    +1     
=========================================
+ Hits           306       307    +1     

Impacted Files	Coverage Δ
itemadapter/adapter.py	100.00% <100.00%> (ø)
itemadapter/utils.py	100.00% <100.00%> (ø)

[elacuesta]

Thanks for the contribution. Changes might be ok for pydantic>=2, but we need to keep supporting pydantic<2. I'd recommend something like this set of changes (currently failing except for the py-pydantic1 environment) to test against both versions.