incompatible with Python 3.13 due to pgpy dependency using deprecated imghdr module

The cybersource-rest-client-python package fails to initialize on Python 3.13 due to a transitive dependency issue with the pgpy package.
See also: https://github.com/SecurityInnovation/PGPy/issues/462

Environment

• Package version: 0.0.70 (latest)
• Python version: 3.13.x
• Operating System: All platforms

Root Cause

1. Python 3.13 removed the deprecated imghdr module (https://peps.python.org/pep-0594/)
2. The pgpy package (v0.6.0) imports imghdr in its constants.py file
3. cybersource-rest-client-python depends on pgpy (defined in setup.py)
4. BatchUploadWithMTLSApi is imported in the top-level CyberSource/__init__.py
5. This triggers the import chain: CyberSource → BatchUploadWithMTLSApi → PgpEncryption → pgpy → imghdr ❌

Error Traceback

  import CyberSource

  File "/site-packages/CyberSource/__init__.py", line 1559, in <module>
      from .api.o_auth_api import OAuthApi
  File "/site-packages/CyberSource/api/__init__.py", line 5, in <module>
      from .batch_upload_with_mtls_api import BatchUploadWithMTLSApi
  File "/site-packages/CyberSource/api/batch_upload_with_mtls_api.py", line 6, in <module>
      from CyberSource.utilities.pgpBatchUpload.pgp_encryption import PgpEncryption
  File "/site-packages/CyberSource/utilities/pgpBatchUpload/pgp_encryption.py", line 4, in <module>
      import pgpy
  File "/site-packages/pgpy/__init__.py", line 4, in <module>
      from .pgp import PGPKey
  File "/site-packages/pgpy/pgp.py", line 25, in <module>
      from .constants import CompressionAlgorithm
  File "/site-packages/pgpy/constants.py", line 5, in <module>
      import imghdr
  ModuleNotFoundError: No module named 'imghdr'

Impact

• Blocks Python 3.13 adoption for all users of this package
• Affects all users, even those not using batch upload functionality, because the import happens at package initialization

Start with setup.py and the import chain through CyberSource/init.py, CyberSource/api/batch_upload_with_mtls_api.py, and CyberSource/utilities/pgpBatchUpload/pgp_encryption.py. Reproduce the top-level import on Python 3.13 and determine the smallest supported dependency or import change; done means the package initializes without the shown ModuleNotFoundError.