test: Remove python3.5 workaround in authproxy

Also, move the burden of checking for a timeout to the client and
disable the timeout on the server. This should avoid intermittent issues
in slow tests (for example mining_getblocktemplate_longpoll.py, or
feature_dbcrash.py), or possibly when the server is running slow (for
example in valgrind).  There shouldn't be any downside in tests caused
by a high rpcservertimeout.

test/functional/test_framework/util.py

@@ -390,6 +390,8 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
             f.write("[{}]\n".format(chain_name_conf_section))
         f.write("port=" + str(p2p_port(n)) + "\n")
         f.write("rpcport=" + str(rpc_port(n)) + "\n")
+        # Disable server-side timeouts to avoid intermittent issues
+        f.write("rpcservertimeout=99000\n")
         f.write("rpcdoccheck=1\n")
         f.write("fallbackfee=0.0002\n")
         f.write("server=1\n")