Drop support.other.format.python scope name.

We use this awkward scope name instead of perhaps more elegant
“support.other.format.python” because it appears that by default
it gives a better visual result in various systems (e.g. GitHub).
This is due to the fact that “storage.type…” is more likely to have
special highlighting in any given color scheme than “support.other…”

Update constant highlighting rules.

We consider something to be a “special constant” if it starts with
“enough” (2 or more in this case) upper-case letters. Any leading
underscores are ignored for the purpose of this definition. Also any
number of underscores and digits are allowed in between the first 2
upper-case letters. To fully satisfy the requirement, the “leading” 2
upper-case letters must be followed by only upper-case letters or
digits until the first underscore.