Switch to Python 3

Python 2 is EOL in a few days.

context/Dockerfile

@@ -6,12 +6,13 @@ LABEL version="1.0"
 RUN echo "deb http://security.debian.org/ buster/updates main" >> /etc/apt/sources.list
 
 RUN apt-get update \
-    && apt-get -y --no-install-recommends install python-pip \
-    && pip install setuptools \
-    && pip install s3cmd \
+    && apt-get -y --no-install-recommends install \
+        python3 python3-pip \
+    && pip3 install setuptools \
+    && pip3 install s3cmd \
     && s3cmd --version \
-    && pip uninstall -y setuptools \
-    && apt-get remove -y python-pip \
+    && pip3 uninstall -y setuptools \
+    && apt-get remove -y python3-pip \
     && apt-get autoremove -y \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*