Add conditional coverage exclusions to trivial places

[webknjaz]

The patch adds version-dependent pragmas where no actual coverage can be provided.
They sometimes are applied to conditionals checking the deps versions and won't work perfectly but it's a start.

The change also excludes tests marked as xfail from coverage — they are either chronically uncovered up to a certain line or have flaky coverage. Either way, having them in is rather harmful for measurements.

Contributor checklist

• Included tests for the changes.
• A change note is created in changelog.d/ (see changelog.d/README.md for instructions) or the PR text says "no changelog needed".

Maintainer checklist

• If no changelog is needed, apply the bot:chronographer:skip label.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[Copilot]

Pull Request Overview

This PR adds coverage pragmas to version-specific code branches to improve coverage reporting. The pragmas instruct the coverage tool to exclude certain branches from coverage tracking when running on specific Python versions, which helps achieve more accurate coverage metrics since version-specific code paths won't be executed in all test environments.

Key changes:

• Added exclude_also configuration to .coveragerc to exclude @pytest.mark.xfail decorated tests
• Added version-specific coverage pragmas to conditional imports and version checks throughout the codebase
• Added explanatory comments for some coverage exclusions

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
.coveragerc	Added exclude_also configuration to exclude xfail tests from coverage
tests/test_sync.py	Added pragma for Python 3.12 version check
tests/test_cli_compile.py	Added pragmas for version checks and FIXME comment about pip version coverage
tests/conftest.py	Added pragmas for pip and Click version checks with explanatory comment
piptools/utils.py	Added pragmas for Python version checks and pip version checks
piptools/build.py	Added pragmas for Python version-specific imports

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