.clang-format

py3: finish porting iteritems() to pycompat and remove source transformer

This commit finishes porting .iteritems() to pycompat.iteritems()
for the mercurial package.

The translation of .iteritems() to .items() was the last conversion
performed by the source transformer. With the porting to pycompat
complete, we no longer have a need for the source transformer. So
the source transformer has been removed. Good riddance! The code
base is now compatible with Python 2 and Python 3.

For the record, as the person who introduced the source transformer,
it brings me joy to delete it. It accomplished its goal to facilitate
a port to Python 3 without overly burdening people on some painful
low-level differences between Python 2 and 3. It is unfortunate we
still have to wallpaper over many differences with the pycompat
shim. But it is what it is.

Differential Revision: https://phab.mercurial-scm.org/D7015

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