Cert Bundle might also be a directory

[e7p]

When using the requests API with the certificate parameter, it is also possible to enter e.g. /etc/ssl/certs as a certificate bundle and it will result in verifying against all certificates lying in this path.

Though, I would recommend also including os.path.isdir() in the statement.

python-zulip-api/zulip/zulip/__init__.py

Line 492 in e6dff1b

if not os.path.isfile(cert_bundle):

[e7p]

Can be linked to #749