The original get_server_certificate in ssl.py does not support socket timeout,

def get_server_certificate(addr, ssl_version=PROTOCOL_TLS, ca_certs=None):
    """Retrieve the certificate from the server at the specified address,
    and return it as a PEM-encoded string.
    If 'ca_certs' is specified, validate the server cert against it.
    If 'ssl_version' is specified, use it in the connection attempt."""

    host, port = addr
    if ca_certs is not None:
        cert_reqs = CERT_REQUIRED
    else:
        cert_reqs = CERT_NONE
    context = _create_stdlib_context(ssl_version,
                                     cert_reqs=cert_reqs,
                                     cafile=ca_certs)
    with  create_connection(addr) as sock:
        with context.wrap_socket(sock) as sslsock:
            dercert = sslsock.getpeercert(True)
    return DER_cert_to_PEM_cert(dercert)

If a timeout parameter, a sample demo can be here:

>>> import ssl
>>> ssl.get_server_certificate(("www.qq.com", 443), timeout=6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/ssl.py", line 1017, in get_server_certificate
    with closing(create_connection(addr, timeout)) as sock:
  File "/usr/lib/python2.7/socket.py", line 575, in create_connection
    raise err
socket.error: [Errno 101] Network is unreachable

It's too late to land a new feature in 3.7.

New changeset b2fac1afaa7c0d41a263781fcf94d8a92dc31b48 by Zackery Spytz in branch 'master':
bpo-31870: Add a timeout parameter to ssl.get_server_certificate() (GH-22270)
https://github.com/python/cpython/commit/b2fac1afaa7c0d41a263781fcf94d8a92dc31b48

New changeset f05c2aed7e25087122613b51f152919c79641f66 by Christian Heimes in branch 'master':
bpo-31870: Fix test_get_server_certificate_timeout on Windows (GH-25570)
https://github.com/python/cpython/commit/f05c2aed7e25087122613b51f152919c79641f66