feat: generate random port instead of just defaulting to 3939

[nealrichardson]

See https://github.com/posit-dev/posit-sdk-py/pull/429/files#diff-1772f099ae7eb5c34361cf3a63ffcff33806c8c5ec987d035774a1134d5f462dR9

Since we're already running Python here, I'm guessing we don't need to shell out for this.