Fix intermittent "Invalid EC key" error

[kislyuk]

Probably just an error in test setup/RNG:

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ERROR: test_basic_signxml_statements (__main__.TestSignXML)
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Traceback (most recent call last):
  File "/Users/kislyuk/projects/signxml/test/test.py", line 128, in test_basic_signxml_statements
    XMLVerifier().verify(signed_data, **verify_kwargs)
  File "/Users/kislyuk/projects/signxml/signxml/__init__.py", line 963, in verify
    self._verify_signature_with_pubkey(
  File "/Users/kislyuk/projects/signxml/signxml/__init__.py", line 634, in _verify_signature_with_pubkey
    key = ecpn.public_key(backend=default_backend())
  File "/opt/homebrew/lib/python3.10/site-packages/cryptography/hazmat/primitives/asymmetric/ec.py", line 364, in public_key
    return ossl.load_elliptic_curve_public_numbers(self)
  File "/opt/homebrew/lib/python3.10/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 1313, in load_elliptic_curve_public_numbers
    self._ec_key_set_public_key_affine_coordinates(
  File "/opt/homebrew/lib/python3.10/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 1480, in _ec_key_set_public_key_affine_coordinates
    raise ValueError("Invalid EC key.")
ValueError: Invalid EC key.

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