Set defer_build=True pydantic config, improving import time significantly

[LewisGaul]

Partially addresses #552. This config option is suggested in pydantic/pydantic#6748.

Before:

$time python -c "import python_on_whales"

real    0m0.405s
user    0m0.401s
sys     0m0.023s

After:

$time python -c "import python_on_whales"

real    0m0.206s
user    0m0.206s
sys     0m0.011s

[gabrieldemarmiesse]

I looked into it and indeed it makes a lot of sense. Thanks! We can merge this when the CI is fixed.