More idiomatic python3 + fix some tests

Hello,

patch 1 to 6 generated mostly using 2to3:
0001: https://docs.python.org/3.8/library/2to3.html#2to3fixer-asserts
0002: https://docs.python.org/3.8/library/2to3.html#2to3fixer-filter
0003: https://docs.python.org/3.8/library/2to3.html#2to3fixer-idioms
0004: https://docs.python.org/3.8/library/2to3.html#2to3fixer-metaclass
0005: https://docs.python.org/3.8/library/2to3.html#2to3fixer-xrange
0006: https://docs.python.org/3.8/library/2to3.html#2to3fixer-unicode

patch 7 fixes change_password tests:
Less that min length : min is 8 instead of 6
Valid password : use an user different than admin user / mock mail module

patch 8 fixes ConstraintDeleteMultipleTestCase on py3.10 (using simplejson
like in all other modules).

Regards,
Cyril