Make click optional for library installs

[lewbei]

Fixes #26.

This follows the maintainer checklist from the issue:

• removes click from the base runtime requirements
• adds a cli extra via extras_require
• bumps the package version to the next major version, 4.0.0
• documents pip install x-wr-timezone[cli] for command line usage

Implementation details:

• importing x_wr_timezone no longer requires click
• when click is installed, x_wr_timezone.main remains a Click command object, preserving the existing CLI tests and entry point behavior
• when click is missing, the CLI entry point exits with a clear install hint

Verification:

$env:Path = "$PWD\.venv\Scripts;$env:Path"
.\.venv\Scripts\python -m pytest

Result: 162 passed, 188 skipped, 2 warnings.

Additional checks:

& 'C:\Program Files\Git\bin\bash.exe' test/test_code_quality.sh
.\.venv\Scripts\python -m pip check
.\.venv-no-click\Scripts\python -m pip check

No-CLI install smoke:

.\.venv-no-click\Scripts\python -m pip install -e .
$env:Path = "$PWD\.venv-no-click\Scripts;$env:Path"
.\.venv-no-click\Scripts\python -c "import importlib.util, x_wr_timezone; print('click', importlib.util.find_spec('click')); print('has_to_standard', hasattr(x_wr_timezone, 'to_standard'));"
x-wr-timezone --help

Observed click None, has_to_standard True, and the CLI prints the pip install x-wr-timezone[cli] hint when click is not installed.

[niccokunzmann]

Thanks for the PR!

There are merge conflicts now. Could you solve them?
It can be merged then.