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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif