Fix download file with special characters

Add test to make sure Sphinx uses proper URL encoding for
special characters in file names.

Use test_html_download() since it test_html5_output() doesn't
quite seem to have easy pattern to verify the contents of href
attribute.

Issue #3097

Signed-off-by: Johannes Aalto <ext-johannes.aalto@vaisala.com>

In case of the descendants of generic class, the value of
obj.__orig_bases__ is incorrect because it returns original base
arguments for the child of the generic class instead of the target
class itself.

This uses obj.__dict__ to get the correct __orig_bases__ information.

Fixes sphinx-doc/sphinx#9564.

The `MethodDocumenter.get_doc` method added by
51ae283a38ed297df3fd5e0554dcc9acaa103e8c prevents
DocstringSignatureMixin from working correctly for __init__ and
__new__ methods.  Additionally, the __new__ docstring was not obtained
correctly.

This commit checks for `self._new_docstrings` being set, and also
corrects the logic for obtaining the __new__ docstring.

There still remains the issue that when the class signature is
obtained from the signature of __init__ or __new__, only the real
signature is used, due to the use of `sphinx.util.inspect.signature`;
the autodoc_docstring_signature option does not have any effect.