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/ebooks/txt/textileml.py

@@ -5,8 +5,10 @@ import re
 
 from functools import partial
 
+from ebook_converter import constants as const
 from ebook_converter.ebooks.htmlz.oeb2html import OEB2HTML
-from ebook_converter.ebooks.oeb.base import XHTML, XHTML_NS, barename, namespace, rewrite_links
+from ebook_converter.ebooks.oeb.base import XHTML, barename, namespace, \
+        rewrite_links
 from ebook_converter.ebooks.oeb.stylizer import Stylizer
 from ebook_converter.ebooks import unit_convert
 from ebook_converter.ebooks.textile.unsmarten import unsmarten
@@ -225,9 +227,9 @@ class TextileMLizer(OEB2HTML):
 
         # We can only processes tags. If there isn't a tag return any text.
         if not isinstance(elem.tag, (str, bytes)) \
-           or namespace(elem.tag) != XHTML_NS:
+           or namespace(elem.tag) != const.XHTML_NS:
             p = elem.getparent()
-            if p is not None and isinstance(p.tag, (str, bytes)) and namespace(p.tag) == XHTML_NS \
+            if p is not None and isinstance(p.tag, (str, bytes)) and namespace(p.tag) == const.XHTML_NS \
                     and elem.tail:
                 return [elem.tail]
             return ['']