Does not ignore unknown service types

[Scott Kitterman]

From RFC 6376:

   s= Service Type (plain-text; OPTIONAL; default is "*"). A colon-
      separated list of service types to which this record applies.
      Verifiers for a given service type MUST ignore this record if the
      appropriate type is not listed. Unrecognized service types MUST
      be ignored. Currently defined service types are as follows:

      * matches all service types

      email electronic mail (not necessarily limited to SMTP)

      This tag is intended to constrain the use of keys for other
      purposes, should use of DKIM be defined by other services in the
      future.

dkimpy doesn't ignore unknown service types. Now that RFC 8460 has defined a new service type, we really ought to do this.

[Scott Kitterman (kitterman)]

2019-10-07 Version 0.9.5
    - Ignore unknown service types in key records (LP: #1847020)
      - This is required by RFC 6376 and predecessors. It becomes important
        now that RFC 8460, which defines a new DKIM service type exists. This
        change is required to avoid processing tlsrpt keys like regular email
        keys, which is incorrect, they have different requirements.