Add support for PEP 440 version specifiers in the --python flag.

[rahuldevikar]

Add PEP 440 version specifier support for --python flag

Implements support for using PEP 440 version specifiers with the --python flag,
allowing users to specify Python versions using operators like >=, <=, ~=, etc.

Examples:

• virtualenv --python='>=3.12' myenv
• virtualenv --python='python>=3.11' myenv
• virtualenv --python='cpython~=3.11.0' myenv

Changes:

• Added SPECIFIER_PATTERN regex to match version specifiers in py_spec.py
• Extended PythonSpec to parse and store version_specifier parameter
• Updated PythonInfo.satisfies() to check version specifier compatibility
• Made packaging library imports optional with graceful fallback
• Updated --python help text with version specifier examples
• Added comprehensive unit and integration tests

Fixes #2994

Use any Python >= 3.12

virtualenv --python=">=3.12" myenv

Use CPython ~= 3.11.0 (compatible release)

virtualenv --python="cpython~=3.11.0" myenv

Use Python between 3.10 and 3.13

virtualenv --python=">=3.10,<=3.13" myenv

With implementation prefix

virtualenv --python="python>=3.11" myenv

[rahuldevikar]

Test succeeding(one timeout): https://github.com/rahuldevikar/virtualenv/actions/runs/20672977256

New run(increased timeout): https://github.com/rahuldevikar/virtualenv/actions/runs/20673442400

[gaborbernat]

CI is also failing.

[gaborbernat]

Released via https://github.com/pypa/virtualenv/releases/tag/20.36.0