Update classifiers and README about Python 3

README.rst

@@ -23,7 +23,7 @@ The idea behind Sigal is to ease the use of the javascript libraries like
 galleria_. These libraries do a great job to display the images, Sigal does
 what is missing: resize images, create thumbnails, generate HTML pages.
 
-Sigal is compatible with Python 2.7 and 3.3+.
+Sigal is compatible with Python 3.5+.
 
 Links :
 

setup.py

@@ -49,6 +49,7 @@ setup(
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Topic :: Internet :: WWW/HTTP',
         'Topic :: Multimedia :: Graphics :: Viewers',
         'Topic :: Software Development :: Libraries :: Python Modules',