Attempt at making BAT work on Windows

All BlendPath instances will use forward slashes, and there should be
more use of PurePosixPath instead of Path.

tests/test_bpathlib.py

@@ -1,4 +1,4 @@
-from pathlib import Path
+from pathlib import Path, PurePosixPath
 import unittest
 
 from blender_asset_tracer.bpathlib import BlendPath
@@ -6,11 +6,12 @@ from blender_asset_tracer.bpathlib import BlendPath
 
 class BlendPathTest(unittest.TestCase):
     def test_string_path(self):
-        p = BlendPath(Path('//some/file.blend'))
+        p = BlendPath(PurePosixPath('//some/file.blend'))
+        self.assertEqual('//some/file.blend', str(PurePosixPath('//some/file.blend')))
         self.assertEqual(b'//some/file.blend', p)
 
         p = BlendPath(Path(r'C:\some\file.blend'))
-        self.assertEqual(b'C:\\some\\file.blend', p)
+        self.assertEqual(b'C:/some/file.blend', p)
 
     def test_is_absolute(self):
         self.assertFalse(BlendPath(b'//some/file.blend').is_absolute())

tests/test_pack.py

@@ -162,7 +162,7 @@ class PackTest(AbstractPackTest):
     def test_execute_rewrite(self):
         infile, _ = self._pack_with_rewrite()
 
-        extpath = pathlib.Path('//_outside_project', *self.blendfiles.parts[1:])
+        extpath = pathlib.PurePosixPath('//_outside_project', *self.blendfiles.parts[1:])
         extbpath = bpathlib.BlendPath(extpath)
 
         # Those libraries should be properly rewritten.
@@ -209,8 +209,9 @@ class PackTest(AbstractPackTest):
         seq = ed.get_pointer((b'seqbase', b'first'))
         seq_strip = seq.get_pointer(b'strip')
 
-        as_bytes = str((self.blendfiles / 'imgseq').absolute()).encode()
-        relpath = b'//_outside_project%b' % as_bytes
+        imgseq_path = (self.blendfiles / 'imgseq').absolute()
+        as_bytes = str(imgseq_path.relative_to(imgseq_path.anchor)).encode()
+        relpath = bpathlib.BlendPath(b'//_outside_project') / as_bytes
 
         # The image sequence base path should be rewritten.
         self.assertEqual(b'SQ000210.png', seq[b'name'])