Accept a major-only python_version such as "3"

[dchaudhari7177]

Summary

Fixes #6687. python_version = "3" in the Pipfile [requires] section — which the docs list as valid — produced a spurious mismatch warning on install and a fatal DeployException under --deploy.

_python_version_matches_required branches on how many components the requirement has, but only distinguished "3 or more" from "everything else":

if len(required_ver_str.split(".")) >= 3:
    return actual == required            # full version, exact
else:
    return actual.major == required.major and actual.minor == required.minor

"3" has one component, so it took the second branch — and parse_version("3") fills in a zero minor, so the comparison became actual.minor == 0. Every 3.x interpreter except 3.0 was rejected:

parse_version("3") -> 3 | major 3 | minor 0
actual=3.13.14  required='3'  -> False

Change

Compare only the components actually given:

requirement	compared
"3"	major only
"3.13"	major + minor
"3.13.1"	exact

The PEP 440 specifier path (">=3.9", ">=3.9,<4") is untouched.

Tests

TestPythonVersionMatchesRequired already has a parametrized table, so the major-only cases go in alongside the existing #6514 substring-regression rows: 3.13.14/3.9.1/3.0.0/3.11 all satisfy "3", while 2.7.18 and 4.0.0 don't, plus 2.7.18 against "2" so the rule isn't special-cased to Python 3.

Reverting only pipenv/ fails exactly the new rows and nothing else:

FAILED ...test_version_match[3.13.14-3-True]
FAILED ...test_version_match[3.9.1-3-True]
FAILED ...test_version_match[3.11-3-True]
FAILED ...test_version_match[2.7.18-2-True]
4 failed, 22 passed

Verification

• pytest tests/unit/test_utils.py -k PythonVersionMatchesRequired → 26 passed
• pytest tests/unit/test_utils.py → 210 passed, 5 skipped (skips are POSIX-only path tests, I'm on Windows)
• ruff check on both changed files → all checks passed
• Added news/6687.bugfix.rst

One note: ruff format --check reports both changed files as needing reformatting, but that's pre-existing on a clean tree (verified by stashing), so I left it alone rather than mixing unrelated reformatting into this diff.

[Copilot]

Pull request overview

This PR fixes incorrect Python version compatibility checks when python_version in the Pipfile [requires] section is specified as a major-only version like "3", aligning behavior with documented support and preventing spurious mismatch warnings / DeployException failures under --deploy.

Changes:

• Update _python_version_matches_required() to compare only the components explicitly provided ("3" → major-only, "3.13" → major+minor, "3.13.1" → exact).
• Extend unit tests to cover major-only python_version requirements (including non-3 majors).
• Add a news/ bugfix entry describing the regression and fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
pipenv/utils/project.py	Adjusts version-matching logic to handle major-only requirements correctly.
tests/unit/test_utils.py	Adds regression tests validating major-only version matching behavior.
news/6687.bugfix.rst	Documents the restored support and user-facing impact.

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