Deprecation of Python 3.6

[kba]

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Python 3.6 will reach end-of-life at the end of 2021 and more and more libraries and tools that we rely on in OCR-D stop supporting Python 3.6 in favor of Python 3.7+. Currently, Ubuntu 18 with Python 3.6 is our target platform, but we want to move to Python 3.7 as soon as possible.

AFAICT today, most OCR-D users have moved on to Python 3.7. So we will drop support for Python 3.6 on September 30, 2021.

If you are using OCR-D with Python 3.6 and cannot easily upgrade, please get in touch by answering on this issue or in our gitter chat.

[bertsky]

For the grace period, I suggest building Python 3.7 images in addition to the current Python 3.6 based images for exhaustive testing in parallel to normal updates.

Changes:

• Makefile: change deps-ubuntu to be responsive to PYTHON envvar (so that make deps-ubuntu PYTHON=python3.7 will apt-get install python3.8 python3-pip python3-venv)
• .circleci/config.yml: additional build config deploy37 which runs make with a PYTHON=python3.7 DOCKER_TAG=ocrd/core3.7 (and DOCKER_TAG=ocrd/core3.7-cuda) override and thus produces ocrd/core37 (and ocrd/core37-cuda)
• in ocrd_all: additional config which this as DOCKER_BASE_IMAGE and builds ocrd/all37:{mini,medi,max}mum{,-cuda{,-git}}

(One could also move the Python version tag from the image name into the image version, i.e. ocrd/core:3.7 / ocrd/all:3.7, but I think this is not the best choice.)