py: Optimise types for common case where type has a single parent type.
The common cases for inheritance are 0 or 1 parent types, for both built-in types (eg built-in exceptions) as well as user defined types. So it makes sense to optimise the case of 1 parent type by storing just the type and not a tuple of 1 value (that value being the single parent type). This patch makes such an optimisation. Even though there is a bit more code to handle the two cases (either a single type or a tuple with 2 or more values) it helps reduce overall code size because it eliminates the need to create a static tuple to hold single parents (eg for the built-in exceptions). It also helps reduce RAM usage for user defined types that only derive from a single parent. Changes in code size (in bytes) due to this patch: bare-arm: -16 minimal (x86): -176 unix (x86-64): -320 unix nanbox: -384 stmhal: -64 cc3200: -32 esp8266: -108