python38: added version 3.8.0

Major new features of the 3.8 series, compared to 3.7

PEP 572, Assignment expressions
PEP 570, Positional-only arguments
PEP 587, Python Initialization Configuration (improved embedding)
PEP 590, Vectorcall: a fast calling protocol for CPython
PEP 578, Runtime audit hooks
PEP 574, Pickle protocol 5 with out-of-band data
Typing-related: PEP 591 (Final qualifier), PEP 586 (Literal types), and PEP 589 (TypedDict)
Parallel filesystem cache for compiled bytecode
Debug builds share ABI as release builds
f-strings support a handy = specifier for debugging
continue is now legal in finally: blocks
on Windows, the default asyncio event loop is now ProactorEventLoop
on macOS, the spawn start method is now used by default in multiprocessing
multiprocessing can now use shared memory segments to avoid pickling costs between processes
typed_ast is merged back to CPython
LOAD_GLOBAL is now 40% faster
pickle now uses Protocol 4 by default, improving performance

(adam)