Fix import of pydantic

[mattt]

Resolves #195

Pydantic 2 introduced some breaking changes, but we work around that by attempting to import its v1 compatibility shim (#121):

try:
    from pydantic import v1 as pydantic
except ImportError:
    import pydantic

However, we had pass instead of import pydantic in a few places, which caused pydantic imports to fail for anyone using pydantic 1.