Link to the correct PEP source file.

Some PEPs are using the .rst extension, and many are in .txt.
The source link currently assumes that all the PEPs are using
the .txt extension.

Fixes python/peps#431

Author: Mariatta

peps/converters.py

@@ -181,7 +181,14 @@ def convert_pep_page(pep_number, content):
 
     data['content'] = str(pep_content)
 
-    source_link = "https://github.com/python/peps/blob/master/pep-{0}.txt".format(pep_number)
+    pep_ext = ".txt"
+    pep_rst_source = os.path.join(settings.PEP_REPO_PATH,
+                            'pep-{}.rst'.format(pep_number))
+    if os.path.exists(pep_rst_source):
+        pep_ext = ".rst"
+
+    source_link = "https://github.com/python/peps/blob/master/pep-{0}{1}".format(
+        pep_number, pep_ext)
     data['content'] += """Source: <a href="{0}">{0}</a>""".format(source_link)
     return data