3.1.11 Revoked objects 
Revoked objects have the following methods:
- all_reasons()
- 
Return a list of all supported reasons.
- get_reason()
- 
Return the revocation reason as a str.  Can be
None, which differs from "Unspecified".
- get_rev_date()
- 
Return the revocation date as a str.
The string is formatted as an ASN1 GENERALIZEDTIME.
- get_serial()
- 
Return a str containing a hex number of the serial of the revoked certificate.
- set_reason(reason)
- 
Set the revocation reason.  reason must
be None or a string, but the values are limited.  
Spaces and case are ignored.  See all_reasons.
- set_rev_date(date)
- 
Set the revocation date.
The string is formatted as an ASN1 GENERALIZEDTIME.
- set_serial(serial)
- 
serial is a string containing a hex number of the serial of the revoked certificate.