Make _compat_skip_empty_lines a per-instance attribute

[prownd]

INISection._compat_skip_empty_lines was a mutable set defined at class level and never re-assigned in init(), so every section of every INIConfig/RawConfigParser in the process shared the same set.

As a consequence, operations on one parser changed the values returned by another one:

>>> a = RawConfigParser()
>>> a.readfp(StringIO('[a]\nfoo = line1\n\n  line2\n'))
>>> a.get('a', 'foo')
'line1\nline2'
>>> b = RawConfigParser(); b.add_section('b'); b.set('b', 'foo', 'x')
>>> a.get('a', 'foo')
'line1\n\nline2'

Create the set in INISection.init() and keep only an immutable placeholder on the class.