Update README install instructions to use pipx

[CodeBySayak]

Summary

This PR updates the installation instructions in README.md to recommend pipx instead of pip3.

Changes

• Replaced pip3 install linode-cli with pipx install linode-cli
• Added pipx upgrade linode-cli command
• Added explanation about why pipx is recommended (PEP 668)

Why

Using pipx avoids conflicts with system-managed Python environments and follows modern Python best practices.

How to Test

• Follow updated install instructions using pipx
• Verify CLI installs and runs correctly

[Copilot]

Pull request overview

This PR updates the README installation guidance to recommend installing linode-cli via pipx instead of pip3, aligning with modern Python packaging practices (e.g., PEP 668 concerns around system-managed environments).

Changes:

• Switched install command from pip3 install linode-cli to pipx install linode-cli.
• Added an upgrade command (pipx upgrade linode-cli) and rationale for using pipx.

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[Copilot]

The new "To upgrade" snippet opens a fenced code block but never closes it, which causes the recommendation text and the following "Visit the Wiki" line to render as code. Close the fence after the upgrade command, and consider using ```bash for consistency with the install snippet, with the explanatory sentence as a normal paragraph outside the code block.

[mgwoj]

Suggested change

	pipx upgrade linode-cli
	pipx upgrade linode-cli
	```