Pure python EC

This removes the dependency on OpenSSL for the interaction tests, by providing a pure-Python
toy implementation of secp256k1.

test/lint/lint-python-dead-code.sh

@@ -15,5 +15,5 @@ fi
 
 vulture \
     --min-confidence 60 \
-    --ignore-names "argtypes,connection_lost,connection_made,converter,data_received,daemon,errcheck,get_ecdh_key,get_privkey,is_compressed,is_fullyvalid,msg_generic,on_*,optionxform,restype,set_privkey,profile_with_perf" \
+    --ignore-names "argtypes,connection_lost,connection_made,converter,data_received,daemon,errcheck,is_compressed,is_valid,verify_ecdsa,msg_generic,on_*,optionxform,restype,profile_with_perf" \
     $(git ls-files -- "*.py" ":(exclude)contrib/" ":(exclude)test/functional/data/invalid_txs.py")