Feature/add choice parameter and raise an exception if fetched value is not within

[Swannbm]

This PR addresses issue #193 (specifying allowed options)

Use case 1:

We don't want to hide warnings because a log-reading probe catches them and triggers an alert.

env = Env(LOGGING_LEVEL="EXCEPTION")
LOGGING_LEVEL = env.str("LOGGING_LEVEL")  # Ok
LOGGING_LEVEL = env.str("LOGGING_LEVEL", choices=["DEBUG", "INFO", "WARNING"])  # raise ImproperlyConfigured

Use case 2:

Define a set of execution environments with dedicated settings, and ensure the app is running in one of them.

env = Env(ENVIRONMENT="prod")
ENVIRONMENT = env.str("ENVIRONMENT")  # Ok
ENVIRONMENT = env.str("ENVIRONMENT", choices=["local", "qa", "staging", "production"])  # raise ImproperlyConfigured

Solution proposed:

• The use cases considered only environment variables of type str.
• Add a choices parameter to Env.str() to allow the user to define allowed options.
• Ensure that the fetched value is included in the provided choices list when the choices parameter is set.

---

I have limited the scope of work, but I can extend the mechanism to some other fetching methods if you see use cases...

Let me know if some corrections are needed.

[Swannbm]

@joke2k hello ! Have you some time to review the proposed PR ? Would be much appreciate.

[coveralls]

coverage: 96.759% (+0.02%) from 96.743%
when pulling fa790df on Swannbm:feature/add-choice-parameter-and-raise-an-exception-if-fetched-value-is-not-within
into c441413 on joke2k:develop.

[joke2k]

Thank you @Swannbm for the contribution and collaboration on this feature. Really appreciated by the maintainers and community. 🙌