[Python binding] Fix setup.py to allow installing in a virtualenv

In a virtualenv:
- site.getusersitepackages() won't import
- get_python_lib() will return the directory inside the virtualenv
- the "--user" option can therefore be safely ignored.

bindings/python/setup.py

@@ -6,7 +6,6 @@ import shutil
 import stat
 import sys
 
-from site import getusersitepackages
 from distutils import log
 from distutils import dir_util
 from distutils.command.build_clib import build_clib
@@ -26,10 +25,14 @@ if os.path.exists(PATH_LIB64) and os.path.exists(PATH_LIB32):
 VERSION = '3.0.3'
 SYSTEM = sys.platform
 
+# virtualenv breaks import, but get_python_lib() will work.
+SITE_PACKAGES = os.path.join(get_python_lib(), "capstone")
 if "--user" in sys.argv:
-    SITE_PACKAGES = os.path.join(getusersitepackages(), "capstone")
-else:
-    SITE_PACKAGES = os.path.join(get_python_lib(), "capstone")
+    try:
+        from site import getusersitepackages
+        SITE_PACKAGES = os.path.join(getusersitepackages(), "capstone")
+    except ImportError:
+        pass
 
 
 SETUP_DATA_FILES = []