Shot in the near dark here: try installing python3-openssl? Let me try installing it here, I'll let you know how it goes.
EDIT: Here's mine
$ pip install https://pxo.nottheeye.com/files/test/knossos/knossos-0.15.0.dev0+73b131a.tar.gz
Traceback (most recent call last):
File "/usr/bin/pip", line 33, in <module>
sys.exit(load_entry_point('pip==20.3.4', 'console_scripts', 'pip')())
File "/usr/bin/pip", line 25, in importlib_load_entry_point
return next(matches).load()
File "/usr/lib/python3.9/importlib/metadata.py", line 77, in load
module = import_module(match.group('module'))
File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 790, in exec_module
File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
File "/usr/lib/python3/dist-packages/pip/_internal/cli/main.py", line 10, in <module>
from pip._internal.cli.autocompletion import autocomplete
File "/usr/lib/python3/dist-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>
from pip._internal.cli.main_parser import create_main_parser
File "/usr/lib/python3/dist-packages/pip/_internal/cli/main_parser.py", line 7, in <module>
from pip._internal.cli import cmdoptions
File "/usr/lib/python3/dist-packages/pip/_internal/cli/cmdoptions.py", line 23, in <module>
from pip._vendor.packaging.utils import canonicalize_name
File "/usr/lib/python3/dist-packages/pip/_vendor/__init__.py", line 94, in <module>
vendored("requests.packages.urllib3.contrib.pyopenssl")
File "/usr/lib/python3/dist-packages/pip/_vendor/__init__.py", line 36, in vendored
__import__(modulename, globals(), locals(), level=0)
File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 664, in _load_unlocked
File "<frozen importlib._bootstrap>", line 627, in _load_backward_compatible
File "<frozen zipimport>", line 259, in load_module
File "/usr/share/python-wheels/urllib3-1.26.5-py2.py3-none-any.whl/urllib3/contrib/pyopenssl.py", line 50, in <module>
File "/usr/lib/python3/dist-packages/OpenSSL/__init__.py", line 8, in <module>
from OpenSSL import crypto, SSL
File "/usr/lib/python3/dist-packages/OpenSSL/crypto.py", line 1556, in <module>
class X509StoreFlags(object):
File "/usr/lib/python3/dist-packages/OpenSSL/crypto.py", line 1577, in X509StoreFlags
CB_ISSUER_CHECK = _lib.X509_V_FLAG_CB_ISSUER_CHECK
AttributeError: module 'lib' has no attribute 'X509_V_FLAG_CB_ISSUER_CHECK'