py/compile: Optimize code generated for "except Exc as var:".
py/compile: Optimize code generated for "except Exc as var:".

When such a clause is used (unlike just "except Exc:"), CPython wraps
exception handler body in try-finally block, and in the finally part,
executes "var = None; del var". The initial assignments appears to be
superfluous with the successive "del". It's there only to cover a case
when exeception handler would contain "del var" itself. Thus, compiler-
generated code would recreate the var and then delete again, to avoid
any spurious exception. Such a code would be rare, so avoid generating
assignment unless MICROPY_CPYTHON_COMPAT is defined.

Change-Id: I9a71821e394f87ff4153ae7c016e7d300147c429

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython