Make several changes in the release and CI setup. (#509)

* Make several changes in the release and CI setup.

- Remove .travis/ directory completely.
- Update deploy.py script to tag with major.minor version
  and remove the v in the tag which it didn't before.
- Fix the CircleCI cron setup.

* Update config.yml

* Use buildkit if possible.

* Use machine executor which supports buildkit.

* Remove setup docker step

* Use newer image.

* Use buildkit syntax for caching pip dependencies.

This should significantly speed up the build process by not having
to build python wheels multiple times.

* Use plain output from buildkit

* Make deploy script work with Python 2.7

Also cache python deps across builds.

* Fix more python3 things

* Install dependency in python3

core/Dockerfile

@@ -1,12 +1,12 @@
+# syntax = docker/dockerfile:1.3
 FROM alpine:3.12
 
-MAINTAINER Abhilash Raj
-
 #Add startup script to container
 COPY docker-entrypoint.sh /usr/local/bin/
 
 #Install all required packages, add user for executing mailman and set execution rights for startup script
-RUN apk update \
+RUN --mount=type=cache,target=/root/.cache \
+    apk update \
     && apk add --virtual build-deps gcc python3-dev musl-dev postgresql-dev \
        libffi-dev \
   # psutil needs linux-headers to compile on musl c library.