readObjectHeader: Allow extra whitespace before "obj"

The header being read has the format:

    <idnum> <generation> obj

where `<idnum>` and `<generation>` are integers.
Previously an arbitrary number of spaces was being allowed between `<idnum>` and `<generation>`, but not between `<generation>` and `obj`.
We now allow arbitrary spaces between `<generation>` and `obj`.

PyPDF2/pdf.py

@@ -1722,6 +1722,7 @@ def readObjectHeader(self, stream):
         idnum = readUntilWhitespace(stream)
         extra |= utils.skipOverWhitespace(stream); stream.seek(-1, 1)
         generation = readUntilWhitespace(stream)
+        extra |= utils.skipOverWhitespace(stream); stream.seek(-1, 1)
         obj = stream.read(3)
         readNonWhitespace(stream)
         stream.seek(-1, 1)