Drops Python 2 support

REP-1030

In addition to some python 2 => 3 fixes, this change bumps the scikit-learn
version to latest. The previously pinned version of scikit-learn failed trying
to compile all necessary C modules under python 3.7+ due to included header files
that weren't compatible with C the API implemented in python 3.7+.

Simultaneously, with the restrictive compatibility supported by scikit-learn,
it seemed prudent to drop python 2 support altogether. Otherwise, we'd be stuck
with python 3.4 as the newest possible version we could support.

With this change, tests are currently passing under 3.9.2.

Lastly, imports the original training data. At some point, a new version
of the training data was committed to the repo but no classifier was
trained from it. Using a classifier trained from this new data resulted
in most of the tests failing.

.build/Dockerfile

@@ -0,0 +1,20 @@
+FROM python:3.9-slim-buster AS deps
+
+RUN apt-get update && \
+    apt-get install -y build-essential git curl python3-dev libatlas3-base libatlas-base-dev liblapack-dev libxml2 libxml2-dev libffi6 libffi-dev musl-dev libxslt-dev
+
+FROM deps AS testable
+ARG REPORT_PATH
+
+VOLUME ["/var/mailgun", "/etc/mailgun/ssl", ${REPORT_PATH}]
+
+ADD . /app
+WORKDIR /app
+COPY wheel/* /wheel/
+
+RUN mkdir -p ${REPORT_PATH}
+
+RUN python ./setup.py build bdist_wheel -d /wheel && \
+    pip install --no-deps /wheel/*
+
+ENTRYPOINT ["/bin/sh", "/app/run_tests.sh"]