Add badge generator, python versions badge and build status badge

With the new generate-badge makefile target, you can generate
python-support.svg, which shows the supported python versions.

Makefile

@@ -71,3 +71,6 @@ get-version:
 
 bump-version:
 	./bump_version.py
+
+generate-badge:
+	$(VIRTUALENV)/bin/anybadge --value='3.5 | 3.6 | 3.7 | 3.8' --label python --file python-support.svg --overwrite

README.rst

@@ -3,6 +3,11 @@ Furnace
 
 A lightweight pure-python container implementation.
 
+.. |build_status| image:: https://github.com/balabit/furnace/workflows/build/badge.svg
+.. |python_support| image:: python-support.svg
+
+|build_status| |python_support|
+
 It is a wrapper around the Linux namespace functionality through libc
 functions like ``unshare()``, ``nsenter()`` and ``mount()``. You can
 think of it as a sturdier chroot replacement, where cleanup is easy (no

requirements-dev.in

@@ -3,3 +3,4 @@ flake8
 pycodestyle
 pytest-cov  # Note: do not sort the list: pytest-cov must be before pytest
 pytest
+anybadge

requirements-dev.txt

@@ -4,6 +4,7 @@
 #
 #    pip-compile --no-emit-trusted-host --no-index --output-file=requirements-dev.txt requirements-dev.in
 #
+anybadge==1.6.2           # via -r requirements-dev.in
 attrs==19.3.0             # via pytest
 autopep8==1.5             # via -r requirements-dev.in
 coverage==5.0.3           # via pytest-cov