This commit updates some outdated syntax with the modern alternatives.
The changes were applied automatically using pyupgrade (v2.0.1) with the
following command:

```
pyupgrade --py3-plus **/*.py
```

These updates are purely syntactical and there should be no change in
functionality.

* `Iterator.next()`` has been replaced with `Iterator.__next__()`
* The `object.__unicode__()` method is not used anymore
* Default source encoding has changed from ASCII to UTF-8

Issue #290 gives an example of a wiki with protected pages whose
protection definitions have no 'expiry' key at all. This seems
strange and, on a quick through the mediawiki code, difficult
to achieve, but since there's a live site out there that does it,
and mediawiki *does* seem to have at least some code to handle
such cases (e.g. how the expiry shows as 'indefinite' in the
page information), let's handle it too, by representing this as
None.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

Allow specifying `uselang` with `Page.revisions()` and
`Site.usercontributions()` to get localized parsed comments with
`prop=parsedcomment`.

The rvexpandtemplates parameter has been removed, but we can use
action=expandtemplates instead.

This fixes tests not being added to the source package.

- Most importantly, the 'protectedpage' error was missing in the list.
- Improve string representation of the error.

- When `Site.force_login` is True we can send `assert=true`
  with `edit` actions to have the api reject our edits if
  we got logged out for some reason.
- Adding a new exception `AssertUserFailedError` as a subclass
  of `LoginError`.

New implementation of #126

- Remove `Page.get_expanded()`
- Postpone removal of `Page.edit()` since it's a more popular method.

per #130

Store the results of page.text() operations in a simple cache
dict. This avoids unnecessary remote roundtrips. Cache is
cleared on each successful page.save() operation. cache
argument can be set to 'False' to disable use of the cache.

Prepare for Python 3.x support
- Use print function
- Use compability methods from six

Back-ported to Python 2.6 and 2.7

- This is more flexible than the `Page.get_expanded()` method,
  since it can be combined with the `section` argument as well,
  to get the expanded wikitext of a section.

- Deprecating `Page.get_expanded()`.

- Also fixed a very minor bug where `Page.last_rev_time` was
  not reset when editing new pages after having edited non-new pages.

- Added more Page tests

since the method does not edit the page, but returns the text of the
page. The old method name still works, but issues a DeprecationWarning.
Note that DeprecationWarnings are silent by default, so we might
consider to elevate it in some future version.