Fix Python version < 2.7.9 error

[arschlochnop]

The tuple form has not always been 100 % compatible, as there was a bug in handling of exec in functions with nested functions (issue 21591);
Reference: https://stackoverflow.com/a/41368813

Error info:

Python 2.7.5 (default, Apr  2 2020, 13:16:51) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux2
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>>> from scriptlets import load_scriptlets
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "scriptlets/__init__.py", line 334
    exec(metadata, metadict)
SyntaxError: unqualified exec is not allowed in function 'parse_scriptlet' it contains a nested function with free variables