make str values in compile time DEF env turn into actually usable Unicode strings when compiling under Python 3

For example

    cpdef enum Eggs
        SOFT
        HARD
        SCRAMBLED

produces three constants with int values that print as strings,
and a type Eggs with attributes Eggs.SOFT, etc. and list(Eggs)
giving the set of all enum values.  Instantiating Eggs with a
numeric or string value will return the appropriate constant.

fix bytes literal creation from compile-time DEF expressions (used to become Unicode strings due to missing encoding)

fix crash where Coroutine deallocation could execute Python code to print a warning which could trigger GC and hit the dead Coroutine object

- comma at end of any argument list (even in lambdas)
- "yield from" does not accept implicit tuple arguments