undefined symbol when importing blspy

[bbjubjub2494]

When using pybind11 2.8+, everything compiles but an ImportError is raised at runtime with the message:

File "/nix/store/p3c7js1a70c3v28vhbcnq1fvffmxwx6f-python3-3.9.7/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 666, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 565, in module_from_spec
  File "<frozen importlib._bootstrap_external>", line 1173, in create_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
ImportError: /nix/store/drrypsf90628kpkghkz8aj4vav07jw7j-python3.9-blspy-1.0.6/lib/python3.9/site-packages/blspy.cpython-39-x86_64-linux-gnu.so: undefined symbol: _ZN3bls9GTElement4SIZEE

To my understanding, this is because constant static members such as GTElement::SIZE sometimes require a definition to be provided explicitly in a single translation unit under C++11 (but not C++17, then they are optional) The issue is alleviated by adding those definitions.

This was encountered on NixOS after force-updating pybind11, but seems like a latent bug so it might be good to nip it in the bud now.

NixOS/nixpkgs#144209