Registering a RHEL system via RHSM through a firewall or proxy fails with 'Unable to verify server's identity: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:897)' error

Issue

Registering a RHEL system via RHSM through a firewall or proxy fails with the following error:

Unable to verify server's identity: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:897)

The following backtrace is logged in /var/log/rhsm/rhsm.log:

2021-12-23 16:48:22,591 [ERROR] dnf:1905:MainThread @repolib.py:358 - [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:897)
Traceback (most recent call last):
  File "/usr/lib64/python3.6/site-packages/subscription_manager/repolib.py", line 355, in __init__
    self.override_supported = 'content_overrides' in get_supported_resources(uep=None, identity=self.identity)
  File "/usr/lib64/python3.6/site-packages/subscription_manager/utils.py", line 245, in get_supported_resources
    return cache.read_data(uep, identity)
  File "/usr/lib64/python3.6/site-packages/subscription_manager/cache.py", line 906, in read_data
    current_data = self._sync_with_server(uep=uep, consumer_uuid=identity.uuid)
  File "/usr/lib64/python3.6/site-packages/subscription_manager/cache.py", line 1014, in _sync_with_server
    return uep.get_supported_resources()
  File "/usr/lib64/python3.6/site-packages/rhsm/connection.py", line 990, in get_supported_resources
    self._load_supported_resources()
  File "/usr/lib64/python3.6/site-packages/rhsm/connection.py", line 978, in _load_supported_resources
    resources_list = self.conn.request_get("/")
  File "/usr/lib64/python3.6/site-packages/rhsm/connection.py", line 906, in request_get
    return self._request("GET", method, headers=headers, cert_key_pairs=cert_key_pairs)
  File "/usr/lib64/python3.6/site-packages/rhsm/connection.py", line 932, in _request
    info=info, headers=headers, cert_key_pairs=cert_key_pairs)
  File "/usr/lib64/python3.6/site-packages/rhsm/connection.py", line 728, in _request
    conn.request(request_type, handler, body=body, headers=final_headers)
  File "/usr/lib64/python3.6/http/client.py", line 1269, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/usr/lib64/python3.6/http/client.py", line 1315, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/usr/lib64/python3.6/http/client.py", line 1264, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/lib64/python3.6/http/client.py", line 1040, in _send_output
    self.send(msg)
  File "/usr/lib64/python3.6/http/client.py", line 978, in send
    self.connect()
  File "/usr/lib64/python3.6/http/client.py", line 1437, in connect
    server_hostname=server_hostname)
  File "/usr/lib64/python3.6/ssl.py", line 365, in wrap_socket
    _context=self, _session=session)
  File "/usr/lib64/python3.6/ssl.py", line 776, in __init__
    self.do_handshake()
  File "/usr/lib64/python3.6/ssl.py", line 1036, in do_handshake
    self._sslobj.do_handshake()
  File "/usr/lib64/python3.6/ssl.py", line 648, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:897)