chore: use pip==20.2.2

Author: busunkim96

.kokoro/docker/Dockerfile

@@ -128,7 +128,7 @@ RUN set -ex \
 # 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 10.0.1
+ENV PYTHON_PIP_VERSION 20.2.2
 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" \
   # we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python