feat: add support for Pydantic V2

[ruicaridade]

Description

As it stands, any project using lago-python-client can't upgrade to Pydantic V2 due to version constraints. Fortunately for us, Pydantic V2 ships with V1 to allow for incremental adoption, which allows us to support both V1 and V2 without fundamentally changing the package's code:

try:
    from pydantic.v1 import BaseModel 
except ImportError:
    from pydantic import BaseModel

In addition to this, I changed all the from pydantic import BaseModel imports to pull from base_model instead.

Type of Change

Feature - non-breaking change

Testing

Used pytest to run the tests once for each version. To replicate:

1. Install the project's base dependencies (in a virtualenv ideally).

pip install -e .

2. Install Pydantic V1.

pip install pydantic==1.10

3. Run pytest.

pytest

4. Install Pydantic V2.

pip uninstall pydantic && pip install pydantic==2.6.1

5. Run pytest again.

pytest