Describe the bug
When I don't use any suffix to specify the tag, the function does not convert the environment variables.

To Reproduce
Steps to reproduce the behavior:

1. Provide a YAML example
   config.yaml

ENV:
  NAME: ${ENVIRONMENT:DEV}

2. Run
   parse_config('./config.yaml', default_value="", tag=None)

3. Results
   print(ENV.NAME)
   ${ENVIRONMENT:DEV}

Expected behavior (if os ENVIRONMENT variable not setted)
print(ENV.NAME)
DEV

Notes
If I use:

parse_config('./config.yaml', default_value="", tag=None)

It works, but according to Pylance rules I can't assign "None" to the tag attribute.

"Argument of type "None" cannot be assigned to parameter "tag" of type "str" in function "parse_config" "None" is not assignable to "str"Pylance"

Additional context
Add any other context about the problem here.

• Python version - 3.311
• OS - linux