src/lib/pythongen.ml: Fix InternalError(str(exception)) for Py3

[bernhardkaindl]

Cc: @edwintorok, @MarkSymsCtx, @lindig

In pythongen.ml, fix return InternalError(str(exn)).failure():

For Python3, compat_block defines str = bytes:

  ; Line "if sys.version_info[0] > 2:"
  ; Block [ Line "long = int"; Line "unicode = str"; Line "str = bytes" ]

The easy fix is to get a copy of str and use it to convert exn to str:

get_str = str
if PY3:
    str = bytes
...
    # In the generated code:
    return InternalError(get_str(exn).failure())

(Instead, we could also prepend def get_str(a): return str(a), this is just shorter. It would be fine either way.)

Without this fix, str(Exception()), compat_block causes it to be bytes(Exception()) on Py3. This would raise a TypeError because Exception can't be converted to bytes:

$ python3 -c 'str = bytes;  str(Exception("hi"))'

Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: cannot convert 'Exception' object to bytes

Setting get_str() to be str() for Py2 and Py3 fixes it and gets a str for InternalError.__init__(arg) (which expects a str value as argument).

[edwintorok]

Thanks, this should allow us to finally move to Python3, at least after this change and a small tweak in XAPI the unit tests pass (and after a short transition period drop Python2 support completely, see #148 at which point we can probably make the 'str' to 'bytes' substitution in the OCaml code itself instead of overriding the names like this)