You can get Python3 from the Synology’s package center, but it comes without pip. But you can easily SSH into your NAS with an admin account and run the following commands:
sudo python3 -m ensurepip --upgrade
sudo python3 -m pip install --upgrade pip
You can check the version with:
python3 -m pip -V
You can install things with:
sudo python3 -m pip install name_of_the_thing
Note that “ensurepip” is a standard Python feature, and also works on other devices. I specifically did this on my Synology NAS, though.