Test failures from sizeof str on win64

[Martin Packman]

The following tests fail on win64, all of which are from expecting the sizeof str objects to be 4 bytes larger than they really are:

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FAIL: test_empty_string (meliae.tests.test__scanner.TestSizeOf)
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Traceback (most recent call last):
  File "meliae\tests\test__scanner.py", line 47, in test_empty_string
    self.assertSizeOf(STRING_SCALING, '', extra_size=0+STRING_BASE, has_gc=False)
  File "meliae\tests\test__scanner.py", line 43, in assertSizeOf
    self.assertEqual(expected_size, _scanner.size_of(obj))
AssertionError: 37 != 33

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FAIL: test_long_string (meliae.tests.test__scanner.TestSizeOf)
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Traceback (most recent call last):
  File "meliae\tests\test__scanner.py", line 54, in test_long_string
    extra_size=100*1024+STRING_BASE, has_gc=False)
  File "meliae\tests\test__scanner.py", line 43, in assertSizeOf
    self.assertEqual(expected_size, _scanner.size_of(obj))
AssertionError: 102437 != 102433

======================================================================
FAIL: test_short_string (meliae.tests.test__scanner.TestSizeOf)
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Traceback (most recent call last):
  File "meliae\tests\test__scanner.py", line 50, in test_short_string
    self.assertSizeOf(STRING_SCALING, 'a', extra_size=1+STRING_BASE, has_gc=False)
  File "meliae\tests\test__scanner.py", line 43, in assertSizeOf
    self.assertEqual(expected_size, _scanner.size_of(obj))
AssertionError: 38 != 34

The estimates just need fixing here. The empty string should be sized as follows:

         2.6 2.7
 ILP32 24 21
  LP64 40 37
 LLP64 36 33