Update doppler_secrets.py

[ZAlexanderV]

The current implementation of the get_option_with_fallback method uses os.environ as if it were a callable:

  val = os.environ(env_var)  

However, os.environ is an instance of _Environ, which behaves like a dictionary and is not callable. Attempting to call it results in a TypeError:

TypeError: '_Environ' object is not callable  

This error occurs whenever a lookup parameter (e.g., project, config, name, token, url) is not provided directly to the plugin and is supposed to fall back to an environment variable. In those cases, get_option_with_fallback tries to read from the environment and fails with the above exception, preventing the plugin from working as intended.
To correctly retrieve values from the environment, os.environ must be accessed like a mapping (dictionary), not called like a function.