Use the real constants module.

This is progressing refactor of the calibre code to make it more
readable, and transform it to something more coherent.

In this patch, there are changes regarding imports for some modules,
instead of polluting namespace of each module with some other modules
symbols, which often were imported from other modules. Yuck.

ebook_converter/utils/xml_parse.py

@@ -1,3 +1,7 @@
+import os
+import tempfile
+import unittest
+
 from lxml import etree
 
 
@@ -24,7 +28,6 @@ def safe_xml_fromstring(string_or_bytes, recover=True):
 
 
 def find_tests():
-    import unittest, tempfile, os
 
     class TestXMLParse(unittest.TestCase):
 
@@ -37,9 +40,11 @@ def find_tests():
             os.remove(self.temp_file)
 
         def test_safe_xml_fromstring(self):
-            templ = '''<!DOCTYPE foo [ <!ENTITY e {id} "{val}" > ]><r>&e;</r>'''
+            templ = '<!DOCTYPE foo [ <!ENTITY e {id} "{val}" > ]><r>&e;</r>'
             external = 'file:///' + self.temp_file.replace(os.sep, '/')
-            self.assertEqual(etree.fromstring(templ.format(id='SYSTEM', val=external)).text, 'external')
+            self.assertEqual(etree.fromstring(templ.format(id='SYSTEM',
+                                                           val=external)).text,
+                             'external')
             for eid, val, expected in (
                 ('', 'normal entity', 'normal entity'),
                 ('', external, external),
@@ -50,7 +55,8 @@ def find_tests():
                 ('PUBLIC', external, None),
                 ('PUBLIC', 'http://example.com', None),
             ):
-                got = getattr(safe_xml_fromstring(templ.format(id=eid, val=val)), 'text', None)
+                got = getattr(etree.fromstring(templ.format(id=eid, val=val)),
+                              'text', None)
                 self.assertEqual(got, expected)
 
     return unittest.defaultTestLoader.loadTestsFromTestCase(TestXMLParse)