Remove pkg_resources from client version lookup

[Mirochill]

Purpose

Remove the import-time dependency on pkg_resources, fixing #30 for environments where recent setuptools no longer provides it.

Summary

• replaced pkg_resources.get_distribution(nats-python) with importlib.metadata.version() when available
• kept Python 3.6/3.7 compatibility by trying the importlib_metadata backport if it is already installed
• added a fallback to the current package version string when package metadata cannot be read, so importing pynats from a source tree still works without pkg_resources

Validation

Not run locally.

Static validation performed:

• git diff --check HEAD~1..HEAD
• rg -n pkg_resources|get_distribution|resource_filename|resource_dir pynats tests pyproject.toml README.md (no matches)