Fix deprecated calls for Python 3.9

The array methods fromstring/tostring have been deprecated since Python
3.2. Python 3.9 removes them completely.

This was discovered when trying to build gitdb package for Fedora 33.
https://bugzilla.redhat.com/show_bug.cgi?id=1788660

Author: lubomir

gitdb/pack.py

@@ -247,6 +247,15 @@ def write(self, pack_sha, write):
         return sha
 
 
+def _frombytes(a, s):
+    """Since Python 3.2 fromstring() is just an alias for frombytes() and is
+    deprecated. Python 3.9 drops the alias."""
+    if sys.version_info.major < 3:
+        a.fromstring(s)
+    else:
+        a.frombytes(s)
+
+
 class PackIndexFile(LazyMixin):
 
     """A pack index provides offsets into the corresponding pack, allowing to find
@@ -410,7 +419,7 @@ def offsets(self):
         if self._version == 2:
             # read stream to array, convert to tuple
             a = array.array('I')    # 4 byte unsigned int, long are 8 byte on 64 bit it appears
-            a.fromstring(buffer(self._cursor.map(), self._pack_offset, self._pack_64_offset - self._pack_offset))
+            _frombytes(a, buffer(self._cursor.map(), self._pack_offset, self._pack_64_offset - self._pack_offset))
 
             # networkbyteorder to something array likes more
             if sys.byteorder == 'little':

gitdb/test/lib.py

@@ -147,6 +147,15 @@ def copy_files_globbed(source_glob, target_dir, hard_link_ok=False):
     # END for each file to copy
 
 
+def _tobytes(a):
+    """Since Python 3.2 tostring() is deprecated and an alias for tobytes().
+    Python 3.9 drops the alias."""
+    if sys.version_info.major < 3:
+        return a.tostring()
+    else:
+        return a.tobytes()
+
+
 def make_bytes(size_in_bytes, randomize=False):
     """:return: string with given size in bytes
     :param randomize: try to produce a very random stream"""
@@ -157,7 +166,7 @@ def make_bytes(size_in_bytes, randomize=False):
         random.shuffle(producer)
     # END randomize
     a = array('i', producer)
-    return a.tostring()
+    return _tobytes(a)
 
 
 def make_object(type, data):