Django 3 support

[fmalina]

• fixes issues with migrations and on_delete (fixes makemigrations fail after setting OPENINGHOURS_PREMISES_MODEL #25, on_delete #26)
• adds support for 2.2 and 3.0 (fixes Django 2.2 Support #39)
• drops Python 2 and Django 1 (fixes Use get_model() from django.apps to fix incompatibility with django-q #27)

Supersedes and closes #28, #29, #32, #33, #37

[jayvdb]

WFM

[francoArdiles]

I got an error with the setup encoding in windows with python 3.8, it gets fixed adding the encoding to the descriptions in the file similar to #35

setup( . . . description=open('DESCRIPTION', encoding="utf-8").read(), long_description=open('README.rst', encoding="utf-8").read(), . . . )

[coveralls]

Coverage increased (+0.1%) to 85.668% when pulling fe9321a on fmalina:master into 6bad475 on arteria:master.

[jayvdb]

@fmalina The last commit, fmalina@9d89299, broke my poetry based project which was using fmalina:master.

@arteria, could you please take a look at this, and fast-forward all except the last commit in this PR.

[fmalina]

@jayvdb I am using pip3 install git+git://github.com/fmalina/django-openinghours.git#egg=openinghours and it breaks for me without the last change

[fmalina]

@jayvdb I reverted it and started to use pip3 install git...#egg=django-openinghours and that seems to work