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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
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