Update secp256k1 lib deps for pythonbitcointx1.1.5

Prior to this commit, code for scalar multiplication the module
secp256k1_main in jmbitcoin relied on direct access to the secp256k1
linked library, but the API for accessing that object has now changed in
python-bitcointx with version 1.1.5 (there is now a Secp256k1 data class
with lib and ctx entries, see the Readme of the project for details).
After this commit, we update our code for that API (but do not make
functional changes to Joinmarket itself).

pyproject.toml

@@ -19,7 +19,7 @@ dependencies = [
 
 [project.optional-dependencies]
 jmbitcoin = [
-    "python-bitcointx==1.1.4",
+    "python-bitcointx==1.1.5",
 ]
 jmclient = [
     "argon2_cffi==21.3.0",

src/jmbitcoin/secp256k1_main.py

@@ -5,18 +5,18 @@
 from jmbase import bintohex
 from bitcointx import base58
 from bitcointx.core import Hash
-from bitcointx.core.secp256k1 import _secp256k1 as secp_lib
-from bitcointx.core.secp256k1 import secp256k1_context_verify
+from bitcointx.core.secp256k1 import get_secp256k1
 from bitcointx.core.key import CKey, CKeyBase, CPubKey
 from bitcointx.signmessage import BitcoinMessage
 
 # This extra function definition, not present in the
 # underlying bitcointx library, is to allow
 # multiplication of pubkeys by scalars, as is required
 # for PoDLE.
+secp_obj = get_secp256k1()
 import ctypes
-secp_lib.secp256k1_ec_pubkey_tweak_mul.restype = ctypes.c_int
-secp_lib.secp256k1_ec_pubkey_tweak_mul.argtypes = [ctypes.c_void_p, ctypes.c_char_p, ctypes.c_char_p]
+secp_obj.lib.secp256k1_ec_pubkey_tweak_mul.restype = ctypes.c_int
+secp_obj.lib.secp256k1_ec_pubkey_tweak_mul.argtypes = [ctypes.c_void_p, ctypes.c_char_p, ctypes.c_char_p]
 
 #Required only for PoDLE calculation:
 N = 115792089237316195423570985008687907852837564279074904382605163141518161494337
@@ -159,8 +159,8 @@ def multiply(s: bytes, pub: bytes, return_serialized: bool = True) -> bytes:
 
     privkey_arg = ctypes.c_char_p(s)
     pubkey_buf = pub_obj._to_ctypes_char_array()
-    ret = secp_lib.secp256k1_ec_pubkey_tweak_mul(
-        secp256k1_context_verify, pubkey_buf, privkey_arg)
+    ret = secp_obj.lib.secp256k1_ec_pubkey_tweak_mul(
+        secp_obj.ctx.verify, pubkey_buf, privkey_arg)
     if ret != 1:
         assert ret == 0
         raise ValueError('Multiplication failed')