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3.1.8 X509Extension objects

X509Extension objects have several methods:

`get_critical`()
  Return the critical field of the extension object.

`get_short_name`()
  Retrieve the short descriptive name for this extension.

  The result is a byte string like ```basicConstraints''`. New in version 0.12.

`get_data`()
  Retrieve the data for this extension.

  The result is the ASN.1 encoded form of the extension data as a byte string. New in version 0.12.