STYLE: src/Python: Fix "undefined name" issues

.flake8

@@ -9,9 +9,3 @@ ignore =
     F403,
     # F405 'XXXX' may be undefined, or defined from star imports
     F405,
-    # F821 undefined name 'constant'
-    F821,
-    # F821 undefined name 'os'
-    F821,
-    # F821 undefined name 'unicode'
-    F821,

src/Python/IO/VTPReader.py

 import vtk
+
+filename = "archive.vtp"
+
 reader = vtk.vtkXMLPolyDataReader()
-path = os.path.join(constant.BASE_DIR, "archive.vtp")  # Archive path
-reader.SetFileName(path)
+reader.SetFileName(filename)
 reader.Update()
 
 mapper = vtk.vtkPolyDataMapper()

src/Python/Utilities/Variant.py

@@ -6,7 +6,7 @@ import sys
 if sys.hexversion >= 0x03000000:
     unicodeEtre = '\xeatre'  # unicode in Python 3
 else:
-    unicodeEtre = unicode('\xeatre', 'latin1')
+    unicodeEtre = unicode('\xeatre', 'latin1')  # noqa: F821
 
 #
 # Basic vtkVariant usage