Make regexp string raw to correct its escape sequence usage (GH-371)

Fixes:

    $ python3 -Wd setup.py
    setup.py:117: DeprecationWarning: invalid escape sequence \.
    ...

setup.py

@@ -114,7 +114,7 @@ def static_env_list(name, separator=None):
 
 def setup_extra_options():
     is_interesting_package = re.compile('^(libxml|libxslt|libexslt)$').match
-    is_interesting_header = re.compile('^(zconf|zlib|.*charset)\.h$').match
+    is_interesting_header = re.compile(r'^(zconf|zlib|.*charset)\.h$').match
 
     def extract_files(directories, pattern='*'):
         def get_files(root, dir_path, files):