Here's a demo of adding a timeout. I'm not sure if this has to be done in the simplestreams library or if it can somehow be set by a caller:

$ diff -u /usr/lib/python3/dist-packages/simplestreams/contentsource.py.orig /usr/lib/python3/dist-packages/simplestreams/contentsource.py
--- /usr/lib/python3/dist-packages/simplestreams/contentsource.py.orig	2021-01-29 21:39:55.695029811 +0000
+++ /usr/lib/python3/dist-packages/simplestreams/contentsource.py	2021-01-29 21:40:57.327987600 +0000
@@ -378,7 +378,7 @@
         if headers == {}:
             headers = None
 
-        self.req = requests.get(url, stream=True, auth=auth, headers=headers)
+        self.req = requests.get(url, stream=True, auth=auth, headers=headers, timeout=10)
         self.r_iter = None
         if buflen is None:
             buflen = READ_BUFFER_SIZE

------------------

$ ./repro.py
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/urllib3/contrib/pyopenssl.py", line 485, in wrap_socket
    cnx.do_handshake()
  File "/usr/lib/python3/dist-packages/OpenSSL/SSL.py", line 1915, in do_handshake
    self._raise_ssl_error(self._ssl, result)
  File "/usr/lib/python3/dist-packages/OpenSSL/SSL.py", line 1622, in _raise_ssl_error
    raise WantReadError()
OpenSSL.SSL.WantReadError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 376, in _make_request
    self._validate_conn(conn)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 996, in _validate_conn
    conn.connect()
  File "/usr/lib/python3/dist-packages/urllib3/connection.py", line 366, in connect
    self.sock = ssl_wrap_socket(
  File "/usr/lib/python3/dist-packages/urllib3/util/ssl_.py", line 370, in ssl_wrap_socket
    return context.wrap_socket(sock, server_hostname=server_hostname)
  File "/usr/lib/python3/dist-packages/urllib3/contrib/pyopenssl.py", line 488, in wrap_socket
    raise timeout("select timed out")
socket.timeout: select timed out

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 439, in send
    resp = conn.urlopen(
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 719, in urlopen
    retries = retries.increment(
  File "/usr/lib/python3/dist-packages/urllib3/util/retry.py", line 400, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File "/usr/lib/python3/dist-packages/six.py", line 703, in reraise
    raise value
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 665, in urlopen
    httplib_response = self._make_request(
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 379, in _make_request
    self._raise_timeout(err=e, url=url, timeout_value=conn.timeout)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 330, in _raise_timeout
    raise ReadTimeoutError(
urllib3.exceptions.ReadTimeoutError: HTTPSConnectionPool(host='images.maas.io', port=443): Read timed out. (read timeout=10)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./repro.py", line 8, in <module>
    r = RequestsUrlReader(url)
  File "/usr/lib/python3/dist-packages/simplestreams/contentsource.py", line 381, in __init__
    self.req = requests.get(url, stream=True, auth=auth, headers=headers, timeout=10)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 60, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 646, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 529, in send
    raise ReadTimeout(e, request=request)
requests.exceptions.ReadTimeout: HTTPSConnectionPool(host='images.maas.io', port=443): Read timed out. (read timeout=10)