Issues extending HTTPAdapter to use a single cipher suite

Question

Ideally I want to use pysslscan, but it is not available to me (neither is pyOpenSSL). I found a post doing part of what I want on superuser. So inspired by a blog post and another SO answer, I tried to set up a requests object where I control the cipher suite:

#!/usr/bin/env python

import requests
import os, sys, subprocess

from requests.adapters import HTTPAdapter
from requests.packages.urllib3.util.ssl_ import create_urllib3_context

hurl = "https://localhost:443/manage/health"

#get ciphers
openssl_ciphers = subprocess.check_output(["openssl", "ciphers","ALL:eNULL"])
cipherList=(openssl_ciphers.splitlines())[0].decode("utf-8").split(':')

class CustomAdapter(HTTPAdapter):
    # This is the 2.11 Requests cipher string, containing 3DES.
    CIPHERS = (
        'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+HIGH:'
        'DH+HIGH:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+HIGH:RSA+3DES:!aNULL:'
        '!eNULL:!MD5'
    )
    def setCiphers(self, cipher):
        self.CIPHERS = cipher
    
    def init_poolmanager(self, *args, **kwargs):
        context = create_urllib3_context(ciphers=self.CIPHERS)
        kwargs['ssl_context'] = context
        return super(CustomAdapter, self).init_poolmanager(*args, **kwargs)

    def proxy_manager_for(self, *args, **kwargs):
        context = create_urllib3_context(ciphers=self.CIPHERS)
        kwargs['ssl_context'] = context
        return super(CustomAdapter, self).proxy_manager_for(*args, **kwargs)

for cipher in cipherList:
    x = CustomAdapter()
    x.setCiphers(cipher)
    s = requests.Session()
    s.mount(hurl, x)
    r = s.get(hurl)
    print("%-*s\t%s" % (30, cipher, r.status_code))

But I get 200 on every single request... I sense a flaw in my cipher-suite setting logic. Most likely I am setting it after the calls to the init_poolmanager and proxy_manager_for methods have been performed.

If I change the CIPHERS to this:

    CIPHERS = (
        'NULL-SHA:'
        '!aNULL:'
        '!eNULL:!MD5'
    )

I get ssl.SSLError: ('No cipher can be selected.',) which is essentially what I want.