CONTRIBUTING

util: implement `writelines()` on atomictempfile

With typehints on the vfs objects, pytype will flag this:

    FAILED: /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi
    /usr/bin/python3.8 -m pytype.single
        --imports_info /mnt/c/Users/Matt/hg/.pytype/imports/mercurial.patch.imports
        --module-name mercurial.patch -V 3.7
        -o /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi
        --analyze-annotated --nofail --quick
        /mnt/c/Users/Matt/hg/mercurial/patch.py
    File "/mnt/c/Users/Matt/hg/mercurial/patch.py", line 535, in writerej:
        No attribute 'writelines' on mercurial.util.atomictempfile [attribute-error]
      In Union[
          mercurial.util.atomictempfile,
          mercurial.vfs.checkambigatclosing,
          mercurial.vfs.delayclosedfile,
          mercurial.windows.fdproxy,
          mercurial.windows.mixedfilemodewrapper
      ]

It's not a real problem there (atomictempfile is only created by passing
different args), but it's reasonable for this to implement the function and
behave like a normal file.  There are other functions missing that can be added
if/when needed.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).