Add Django 3.0 support, remove Python 2.7 support

[benjaoming]

#514

TODO:

• Clean up unicode-related stuff from Python 2.7

[codecov-io]

Codecov Report

Merging #515 into master will decrease coverage by 0.49%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #515     +/-   ##
=========================================
- Coverage   98.13%   97.64%   -0.5%     
=========================================
  Files          29       29             
  Lines         913      890     -23     
  Branches      155      155             
=========================================
- Hits          896      869     -27     
- Misses         12       14      +2     
- Partials        5        7      +2

Impacted Files	Coverage Δ
djmoney/forms/widgets.py	87.5% <ø> (ø)	⬆️
djmoney/utils.py	100% <ø> (ø)	⬆️
djmoney/apps.py	100% <ø> (ø)	⬆️
djmoney/models/validators.py	100% <ø> (ø)	⬆️
djmoney/templatetags/djmoney.py	100% <ø> (ø)	⬆️
djmoney/models/__init__.py	100% <ø> (ø)	⬆️
djmoney/settings.py	100% <ø> (ø)	⬆️
djmoney/money.py	97.1% <ø> (-2.9%)	⬇️
djmoney/contrib/django_rest_framework/fields.py	100% <ø> (ø)	⬆️
djmoney/contrib/django_rest_framework/__init__.py	100% <ø> (ø)	⬆️
... and 12 more

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[lundberg]

Lovely PR!

FYI, tests on travis are passing (except linting), but when trying this PR locally in a project, I get an ImportError referring to djmoney.serializers importing django.utils.six here.

Not covered?

[benjaoming]

Thanks @lundberg :) This should be working now.

Moreover, the flake8 command wasn't working in Travis CI.. so important stuff such as undeclared variables was missing in the linting.

@Stranger6667 would you be able to take a look at this and merge at will?

[Stranger6667]

I like this PR :) It is finally happening. Just one question regarding _compat
Thank you, Ben :)

[benjaoming]

@Stranger6667 - the last change was necessary, since we had specified universal=True for building wheels, which says that the .whl will work on both Python 2 and 3.

Now, we just create .whl for Python 3, anyone running pip install django-money on Python 2 will get version 0.15, and Python 3 environments will get version 1.0 (when it's released).