Declare json-patch operations as a class-based attribute

[tzoiker]

I propose to declare available JSON-patch operations as class-based attributes rather than in the constructor.

Motivation:

• It seems a bit more logical.
• It became impossible to add custom operations without complete copy-paste of the constructor due to added validation.

Python 2.7 does not support MappingProxyType, so it should be handled with more care (due to dict mutability), although I suppose it is not much of a problem as python2.7 was deprecated.

[coveralls]

Coverage decreased (-0.1%) to 65.846% when pulling 9310d48 on tzoiker:fix/json-patch-ops into f3c46e8 on stefankoegl:master.

[stefankoegl]

Thanks for your contribution! At first glance it looks like a worthwhile change. However I noticed two issues:

• the tests fail for most python versions
• the diff contains a lot of unrelated changes which should be removed (possible whitespace-only changes)

Please update your pull request accordingly. I'll be able to do a more detailed review afterwards.

Thanks!

[tzoiker]

• the tests fail for most python versions

Fixed.

• the diff contains a lot of unrelated changes which should be removed (possible whitespace-only changes)

I had to reorder JSONPatch and operations, otherwise, they are not found when JSONPatch is declared

[stefankoegl]

I just released version 1.28 with this change - thanks for your contribution!