feat: accept API key as pydantic.SecretStr

[cjumel]

Description

As the title says, the main element of this PR is to support pydantic.SecretStr as API key parameter. Additionally, the corresponding client attribute was also converted to a pydantic.SecretStr to decrease the risk of the API key leaking into logs for instance.

Besides this, this PR does two maintenance things:

• enforce additional linting rules from our internal rule set (all those which could be fixed automatically, the ones requiring manual work are left for the future)
• make internal modules and class variables private with single leading _

Checklist

• I have installed pre-commit on this project (for instance with the make install-dev command)
  before creating any commit, or I have run successfully the make lint command on my changes
• I have run successfully the make test command on my changes
• I have updated the README.md if my changes affected it