pysymbol fails to run on packages that use DepreciationWarning

If `pkgutil.walk_packages()` encounters depreciated modules, the whole conversion fails

# Example

`pysym2md ray` fails because some modules raise `DepreciationWarning`.

PR coming shortly.

Reproduce the failure with `pysym2md ray` and focus on the package discovery path that calls `pkgutil.walk_packages()`. Done means conversion completes when deprecated modules raise `DepreciationWarning` instead of failing for the whole package.

Written by the indexing model from the issue text.