adding optional (not enabled by default) ability to encode keys

[rdyro]

This pull requests adds the optional behavior of encoding keys.

The justification: support for hashable, but not str (or int) keys, e.g. tuple as a key.

The changes include:

• the core class now allows passing encode_key and decode_key method
• both of these methods are the identity function by default
• the module now provides two default encode_key and decode_key which are base64 encoding of pickle dump of key
• the changes do not change the sqlite key type: TEXT
• the changes are tested, I added a test for a tuple key

Potential problems:

• the optional key encoding using pickle serialization, so it allows non-hashable types to act as keys

[mpenkov]

Can you please fix the flake8 errors in the code as well?

https://github.com/RaRe-Technologies/sqlitedict/actions/runs/3303841568/jobs/5456104420

[rdyro]

I just fixed the flake8 errors.

Would documenting this optional key encoding in the README be a good idea? I added a commit for that.

[rdyro]

Could we merge this into master?

This PR adds attempts to add a useful optional feature, while introducing minimum extra complexity and not affecting performance.

[mpenkov]

Yes, of course. I've merged your work. Thank you!