Log Message:
py-pydicom: Update to 1.2.1.

From Eric A. Borisch in pull request NetBSD/pkgsrc#40.

-   Do not derive Dataset from dict (#767)
    -   fixes side effects from initializing with another dataset
-   Added missing dict methods that are passed to the tags dict
-   Adapted documentation to Dataset changes
-   Make sure that the retry order config is reset in the test (#772)