Upgrade pip before in case of very old version on system:
py -3 -m pip install --user -U pip
Upgrade all root packages in one go (avoid generating broken dependencies between root projects):
DEL "%TMP%\pip-upgrade.txt"
FOR /F "delims==" %i IN ('py -3 -m pip list --user --not-required --format=freeze') DO @ECHO %i >>"%TMP%\pip-upgrade.txt"