chore: updates coincurve requirement to v13

Update to v13 as v12 started to raise undefined symbol exceptions agains latest
libsecp256k1.

Note: any version of `pip install coincurve` fails if no libsecp256k1 headers
are installed, should we point this out somewhere/somehow?

Changelog-None

contrib/pyln-proto/requirements.txt

@@ -1,5 +1,5 @@
 bitstring==3.1.6
 cryptography==2.8
-coincurve==12.0.0
+coincurve==13.0.0
 base58==1.0.2
 secp256k1==0.13.2