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/feature_dbcrash.py

@@ -51,9 +51,11 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         self.supports_cli = False
 
         # Set -maxmempool=0 to turn off mempool memory sharing with dbcache
-        # Set -rpcservertimeout=900 to reduce socket disconnects in this
-        # long-running test
-        self.base_args = ["-limitdescendantsize=0", "-maxmempool=0", "-rpcservertimeout=900", "-dbbatchsize=200000"]
+        self.base_args = [
+            "-limitdescendantsize=0",
+            "-maxmempool=0",
+            "-dbbatchsize=200000",
+        ]
 
         # Set different crash ratios and cache sizes.  Note that not all of
         # -dbcache goes to the in-memory coins cache.