Merge patch from Ronny to not compile C extensions when building in pypy.

diff --cc NEWS
index 2d9fb2b9678b235d0d63e0ad0a376defaa9e12e8,2d9fb2b9678b235d0d63e0ad0a376defaa9e12e8..6171b120931a3bc38460cd608028735d290b6f7c
--- 1/NEWS
--- 2/NEWS
+++ b/NEWS
@@@ -32,6 -32,6 +32,9 @@@
    * Smart protocol clients can now change refs even if they are
      not uploading new data. (Jelmer Vernooij, #855993)
  
++ * Don't compile C extensions when running in pypy.
++   (Ronny Pfannschmidt, #881546)
++
   API CHANGES
  
    * ``Repo.revision_history`` is now deprecated in favor of ``Repo.get_walker``.

diff --cc setup.py
index 7472ee66c077b09828002f09b272aea2f1c90ab7,90e5238fc0adc1b0a2717d6cb8d317075b953123..50beb13f28009a763fecf390bb5e044c9ba09abb
--- 1/setup.py
--- 2/setup.py
+++ b/setup.py
@@@ -31,7 -31,7 +31,7 @@@ class DulwichDistribution(Distribution)
  
      global_options = Distribution.global_options + [
          ('pure', None, 
--            "use pure (slower) Python code instead of C extensions")]
++            "use pure Python code instead of C extensions (slower on CPython)")]
  
      pure = False