Make the build reproducible

Whilst working on the Reproducible Builds effort [0] we noticed that
libsass-python could not be built reproducibly.

This is due to the manpage containing content that is generated by
iterating over a Python frozenset type, which is non-deterministic.

This was originally filed in Debian as #971527 [1].

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/971527

sassutils/builder.py

@@ -22,7 +22,7 @@
 
 #: (:class:`re.RegexObject`) The regular expression pattern which matches to
 #: filenames of supported :const:`SUFFIXES`.
-SUFFIX_PATTERN = re.compile('[.](' + '|'.join(map(re.escape, SUFFIXES)) + ')$')
+SUFFIX_PATTERN = re.compile('[.](' + '|'.join(map(re.escape, sorted(SUFFIXES))) + ')$')
 
 
 def build_directory(