chore: Cleanup casters to avoid unnecessary ref counting

[Skylion007]

Description

• I noticed some style inconsistency where we inc_ref() 'None()' when returning them in casters. In reality, we should be calling release() on them to simply return the existing handle from the underlying object.
• In other words, currently we call inc_ref() twice and dec_ref() once to increase the ref count by one (once in ctor, once manually, once in the dor). This change makes it so we only call it once in the ctor.
• This also makes the style more consistent for when people write their own casters.

Suggested changelog entry:

* Optimize casting C++ object to None.