Replace pkg_resources with importlib.metadata for Python 3.12 compati…

[Robolightning]

Motivation

This PR replaces the legacy pkg_resources module with modern importlib.metadata to ensure compatibility with Python 3.12.
pkg_resources is deprecated and no longer installed by default in Python 3.12+, causing installation failures when building from source (see issue #3325).
The change maintains full backward compatibility with Python < 3.8 by falling back to pkg_resources when importlib.metadata is not available.

Modification

• Modified setup.py to conditionally import importlib.metadata (Python ≥ 3.8) or fall back to pkg_resources (Python < 3.8).
• Replaced direct usage of pkg_resources.get_distribution with a wrapper that provides the same interface (get_distribution(name).version).
• Replaced pkg_resources.parse_version with packaging.version.parse (with a fallback for environments where packaging is not installed).
• Added a compatibility layer for DistributionNotFound / PackageNotFoundError.

These changes are isolated to the installation script and do not affect runtime behaviour.

BC-breaking (Optional)

No breaking changes.
The new implementation is fully compatible with older Python versions (3.6+) thanks to the fallback mechanism.
Downstream projects relying on mmcv’s setup.py will not observe any difference.

Use cases (Optional)

Not applicable – this is a maintenance change to enable successful installation under Python 3.12.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.