Update cmd.py, fix PermissionError issue using best practices

This closes gitpython-developers#830

Author: cclauss

git/cmd.py

@@ -43,6 +43,10 @@
     stream_copy,
 )
 
+try:
+    PermissionError
+except NameError:  # Python < 3.3
+    PermissionError = OSError
 
 execute_kwargs = {'istream', 'with_extended_output',
                   'with_exceptions', 'as_process', 'stdout_as_string',
@@ -211,22 +215,14 @@ def refresh(cls, path=None):
 
         # test if the new git executable path is valid
 
-        if sys.version_info < (3,):
-            # - a GitCommandNotFound error is spawned by ourselves
-            # - a OSError is spawned if the git executable provided
-            #   cannot be executed for whatever reason
-            exceptions = (GitCommandNotFound, OSError)
-        else:
-            # - a GitCommandNotFound error is spawned by ourselves
-            # - a PermissionError is spawned if the git executable provided
-            #   cannot be executed for whatever reason
-            exceptions = (GitCommandNotFound, PermissionError)
-
+        # - a GitCommandNotFound error is spawned by ourselves
+        # - a PermissionError is raised if the git executable provided
+        #   cannot be executed for whatever reason
         has_git = False
         try:
             cls().version()
             has_git = True
-        except exceptions:
+        except (GitCommandNotFound, PermissionError):
             pass
 
         # warn or raise exception if test failed