util: move common proxyobserver attributes to the base class Fixes the following pytype warnings: line 791, in _writedata: No attribute 'logdata' on baseproxyobserver [attribute-error] line 792, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error] line 793, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 794, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 799, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error] line 800, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 802, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 803, in _writedata: No attribute 'name' on baseproxyobserver [attribute-error] line 805, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 809, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error] line 810, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 814, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] line 815, in _writedata: No attribute 'name' on baseproxyobserver [attribute-error] line 817, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error] Differential Revision: https://phab.mercurial-scm.org/D7675