Support libsecp256k1 v0.7.0

[3nprob]

Since v0.7.0, libsecp256k1 has renamed secp256k1_ec_privkey_<operation> in favor of secp256k1_ec_seckey_<operation>.

• https://github.com/bitcoin-core/secp256k1/releases
• https://github.com/bitcoin-core/secp256k1/blob/master/CHANGELOG.md

This breaks python-bitcointx:

AttributeError: /usr/lib/libsecp256k1.so: undefined symbol: secp256k1_ec_privkey_tweak_add. Did you mean: 'secp256k1_ec_pubkey_tweak_add'?
Traceback (most recent call last):
...
  File "/usr/local/lib/python3.13/dist-packages/bitcointx/core/secp256k1.py", line 131, in get_secp256k1
    _secp256k1 = secp256k1_load_library(bitcointx.util._secp256k1_library_path)
  File "/usr/local/lib/python3.13/dist-packages/bitcointx/core/secp256k1.py", line 327, in secp256k1_load_library
    cap = _add_function_definitions(handle)
  File "/usr/local/lib/python3.13/dist-packages/bitcointx/core/secp256k1.py", line 202, in _add_function_definitions
    lib.secp256k1_ec_privkey_tweak_add.restype = ctypes.c_int
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Related:

• Issues with configure bitcointx to work with libsecp256k1 #83