setup.py: workaround for missing bz2 module in IronPython

IronPython does not provide the bz2 module on its own.  This patch skips
importing it to allow setup to continue.

(minor tweaks by mpm)

setup.py

@@ -4,7 +4,7 @@
 # 'python setup.py install', or
 # 'python setup.py --help' for more options
 
-import sys
+import sys, platform
 if not hasattr(sys, 'version_info') or sys.version_info < (2, 4, 0, 'final'):
     raise SystemExit("Mercurial requires Python 2.4 or later.")
 
@@ -36,11 +36,21 @@ except:
     raise SystemExit(
         "Couldn't import standard zlib (incomplete Python install).")
 
+# The base IronPython distribution (as of 2.7.1) doesn't support bz2
+isironpython = False
 try:
-    import bz2
+    isironpython = platform.python_implementation().lower().find("ironpython") != -1
 except:
-    raise SystemExit(
-        "Couldn't import standard bz2 (incomplete Python install).")
+    pass
+
+if isironpython:
+    print "warning: IronPython detected (no bz2 support)"
+else:
+    try:
+        import bz2
+    except:
+        raise SystemExit(
+            "Couldn't import standard bz2 (incomplete Python install).")
 
 import os, subprocess, time
 import shutil