typing.Protocol is better candidate for base serialization classes.

[PradyumnaKrishna]

Description of issue or feature request:
The following classes of serialization module would be better candidates for typing.Protocol than abc:

• BaseSerializer
• BaseDeserializer
• JSONSerializable

typing.Protocol requires python version 3.8 and above and hence this couldn't be done now.
After dropping support of python 3.7 by securesystemslib these classes should use typing.Protocol.

Current Behaviour
Mentioned classes are abstract base class in serialization module.

Expected behavior:
Mentioned classes should be typing.Protocol.

Originally posted by @lukpueh in #9 (review)

[adityasaky]

Can we track this in the upstream securesystemslib repository instead of here?

[lukpueh]

I think I am in favor of just closing this issue:

• using Protocol does not improve user/developer experience significantly enough to justify using a backport for Python 3.7 now, or a refactor after dropping 3.7

• @jku raised valid concerns about lack of declaration in classes that implement a Protocol