IronPython: impossible to use pip - python-2.7

Question

In the last two weeks I noticed that my ironpython installation is not able to use pip anymore, e.g.:
ipy.exe -m pip install <package>
the stacktrace is similar to the following:
File "<path_to_bin>\bin\lib\site-packages\pip\basecommand.py", line 215, in main
status = self.run(options, args)
File "<path_to_bin>\bin\lib\site-packages\pip\commands\install.py", line 324, in run
requirement_set.prepare_files(finder)
File "<path_to_bin>\bin\lib\site-packages\pip\req\req_set.py", line 376, in prepare_files
discovered_reqs.extend(self._prepare_file(
File "<path_to_bin>\bin\lib\site-packages\pip\req\req_set.py", line 551, in _prepare_file
req_to_install.populate_link(
File "<path_to_bin>\bin\lib\site-packages\pip\req\req_install.py", line 278, in populate_link
self.link = finder.find_requirement(self, upgrade)
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 465, in find_requirement
all_candidates = self.find_all_candidates(req.name)
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 423, in find_all_candidates
for page in self._get_pages(url_locations, project_name):
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 568, in _get_pages
page = self._get_page(location)
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 683, in _get_page
return HTMLPage.get_page(link, session=self.session)
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 811, in get_page
inst = cls(resp.content, resp.url, resp.headers)
File "<path_to_bin>\bin\lib\site-packages\pip\index.py", line 728, in __init__
self.parsed = html5lib.parse(
File "<path_to_bin>\bin\lib\site-packages\pip\_vendor\html5lib\html5parser.py", line 35, in parse
if self.data.matchBytes(key):
File "<path_to_bin>\bin\lib\site-packages\pip\_vendor\html5lib\_inputstream.py", line 668, in matchBytes
data = self[p:p + len(bytes)]
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
for any package, even trying to update pip itself.
I currently use ironpython 2.7.12, but it seems to fail the same way with the beta 3.4. I install pip with the ensurepip module, which corresponds to pip-19.2.3 setuptools-41.2.0