This PR adds a check to the pip3 step that skips if the file EXTERNALLY-MANAGED is present within python's stdlib folder, indicating that the base environment is managed by an external manager, such as the OS's package manager. Closes #365.

See: PEP 668 – Marking Python base environments as “externally managed”

Standards checklist:

• The PR title is descriptive.
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

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