Modify Python install command in README

[valorisa]

Update Python installation command to check for existing Python version.

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue number)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Documentation update (adding missing information, correcting errors)
• New section added (adding new documentation content)
• Refactoring (no functional changes, no doc changes)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own changes
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings (markdownlint passes)
• I have checked for broken links
• Any dependent changes have been merged and published in downstream modules

Changes Made

Files Modified

Files Added

Files Deleted

Testing

Platforms Tested

• Windows 10/11
• macOS (Intel)
• macOS (Apple Silicon)
• Linux (Ubuntu/Debian)
• Linux (Fedora/RHEL)
• Linux (Arch)

Validation

• markdownlint passes
• All links checked and valid
• Spelling verified

Screenshots (if applicable)

Additional Notes

Add any additional notes for reviewers here.

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