Wikitextparser

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- Fixed some other bugs in ``plain_text``/``remove_markup`` functions for:

- images containing wikitext
- tags containing bold/italic items
- nested tags

- Fixed a bug in extracting sub-tags.

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- Fixed a bug in Tag objects causing strange behaviour upon mutating a tag.
- Fixed a bug in ``plain_text``/``remove_markup`` functions, causing some objects that are expected to be removed, remain in the result. (39)

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- Fix syntax errors for python 3.5, 3.6, and 3.7.

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- Fix a bug in getting the parser functions of a Template object.

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- Fix a catastrophic backtracking issue for wikitexts containing html tags. (37)

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- Add ``wikitextparser.remove_markup`` function and ``WikiText.plain_text`` method.
- Improve detection of parameters and wikilinks.
- Add ``get_bolds`` and ``get_italics`` methods.
- ``WikiLink.wikilinks``, ``WikiList.get_lists()``, ``Template.templates``, ``Tag.get_tags()``, ``ParserFunction.parser_functions``, and ``Parameter.parameters`` won't return objects equal to ``self`` anymore, only sub-elements will be returned.
- Improve handling of comments within wikilinks.
- ``WikiLink.text.setter`` no longer accepts None values. This was marked as deprecated since v0.25.0.
- Drop support for Python 3.4.
- Remove the deprecated ``pprint`` method. Users should use ``pformat`` instead.
- Allow a tuple of patterns in ``get_list`` and ``sublists`` method. The default ``None`` is now deprecated and a tuple is used instead.