import-checker: allow symbol imports from mercurial.pycompat

Currently, the source transformer inserts
`from mercurial.pycompat import delattr, getattr, hasattr, setattr, open, unicode`
to the top of every file. As part of getting rid of the source transformer,
we'll need to have source code call these wrappers directly. Rather than
rewrite all call sites to call pycompat.*, I think it makes sense to import
needed symbols via explicit imports. That requires loosening the import checker
to allow this.

Differential Revision: https://phab.mercurial-scm.org/D7004

contrib/import-checker.py

@@ -30,6 +30,7 b' allowsymbolimports = ('
     'mercurial.i18n',
     'mercurial.interfaces',
     'mercurial.node',
+    'mercurial.pycompat',
     # for revlog to re-export constant to extensions
     'mercurial.revlogutils.constants',
     'mercurial.revlogutils.flagutil',