Fix Python min alignment

This was accidentally deleted in:
9fa8245e81

Change-Id: I8cad721e075279f6a67aca81259f73fb75aba482

python/flatbuffers/builder.py

@@ -514,6 +514,13 @@ class Builder(object):
         """Finish finalizes a buffer, pointing to the given `rootTable`."""
         N.enforce_number(rootTable, N.UOffsetTFlags)
 
+        prepSize = N.UOffsetTFlags.bytewidth
+        if file_identifier is not None:
+            prepSize += N.Int32Flags.bytewidth
+        if sizePrefix:
+            prepSize += N.Int32Flags.bytewidth
+        self.Prep(self.minalign, prepSize)
+
         if file_identifier is not None:
             self.Prep(N.UOffsetTFlags.bytewidth, encode.FILE_IDENTIFIER_LENGTH)