Fix compatibility issues with python 3.10 and django 4.0

[julienc91]

This MR intends to fix multiple compatibility issues.

• The first one is the use of collections.Mapping which has been deprecated since python 3.3, instead of collections.abc.Mapping. In Python 3.10, this won't work anymore.

$ python
Python 3.9.1 (default, Jan 20 2021, 00:00:00) 
[GCC 10.2.1 20201125 (Red Hat 10.2.1-9)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import Mapping
<stdin>:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working

• The second one is a compatibility issue with the upcoming Django 4.0 version. The use of the providing_args parameter for django's Signal object has always been purely documentational, and is planned to be completely removed in Django 4.0.

$ python -W all
Python 3.9.1 (default, Jan 20 2021, 00:00:00) 
[GCC 10.2.1 20201125 (Red Hat 10.2.1-9)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from django.dispatch import Signal
>>> Signal(providing_args=["foo"])
<stdin>:1: RemovedInDjango40Warning: The providing_args argument is deprecated. As it is purely documentational, it has no replacement. If you rely on this argument as documentation, you can move the text to a code comment or docstring.

• The third one is another compatibility issue with Django 4.0. The use of django.conf.urls is deprecated since django 2.0. Also, django.urls.url will be removed from Django 4.0. django.urls.re_path can be used instead, as it was an alias for the same function since django 2.0.

• The last one is another compatibility issue with Django 4.0, but I didn't find any documentation on it. It's due to this commit that modifies the internal attribute name of the FieldFile class.

For each of these issues, I tried to keep full retro-compatibility with the previous version.

[agateblue]

Looks good, thank you!