sslutil: guard against broken certifi installations (issue5406) Certifi is currently incompatible with py2exe; the Python code for certifi gets included in library.zip, but not the cacert.pem file - and even if it were included, SSLContext can't load a cacert.pem file from library.zip. This currently makes it impossible to build a standalone Windows version of Mercurial. Guard against this, and possibly other situations where a module with the name "certifi" exists, but is not usable.