fix: coincurve build issue

hatchling build requires license file but the build that coincurve 
21.0.0 uses doesn’t account for that in `build_hatch.py` this was added 
to prevent need of `cffi` as a runtime dependency but we can probably 
live without it until it gets fixed.

change MR: ofek/coincurve#176
 
reported issue: ofek/coincurve#187

Author: king-11

contrib/pyln-proto/pyproject.toml

@@ -11,7 +11,7 @@ requires-python = ">=3.8,<4.0"
 dependencies = [
     "base58>=2.1.1",
     "bitstring>=4.1.0",
-    "coincurve>=20",
+    "coincurve==20.0.0",
     "cryptography>=42",
     "PySocks>=1"
 ]