chore: use pip 21.3.1 (#177)

https://pypi.org/project/pip/21.3.1/

Author: busunkim96

python/googleapis/python-multi/Dockerfile

@@ -149,7 +149,7 @@ RUN for PYTHON_VERSION in 2.7.18 3.6.15 3.7.12 3.8.12 3.9.7 3.10.0; do \
 # Install pip on Python 3.6 only.
 # If the environment variable is called "PIP_VERSION", pip explodes with
 # "ValueError: invalid truth value '<VERSION>'"
-ENV PYTHON_PIP_VERSION 20.2.4
+ENV PYTHON_PIP_VERSION 21.3.1
 RUN wget --no-check-certificate -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
     && python3.6 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \