Optimzie if vs if-else block generation.

By pushing key/value pairs on the stack and creating dict in one go. The
original Py2 is to create an emoty dict and add items one by one. This
saves stack size, but slower/more bytecode.

…riends.

As required in Python3.

Python3 syntax is supported, where "except" and "finally" can be attached
to the same "try". Internally, such a statement into enclosed try-finally
with try-except inside.

… Py3.

Per https://docs.python.org/3/reference/compound_stmts.html#the-try-statement

except E as N:
    foo

is compiled as:

except E as N:
    try:
        foo
    finally:
        del N

and actually it's even compiled as:

except E as N:
    try:
        del N
    finally:
        N = None
        del N

to allow, as shown, user's exeption handler body to contain "del N"
statement (so, to avoid exception, compiler-generated finally handler
first recreates the var and then deletes it).