Remove Python 3.4 references

This PR removes the Python 3.4 tox task and updates references in the
code to Python 3.5+.

tox complains about Python 3.4, which is EOL after next month:

> py34 installed: DEPRECATION: Python 3.4 support has been deprecated. pip 19.1 will be the last one supporting it. Please upgrade your Python as Python 3.4 won't be maintained after March 2019 (cf PEP 429).

pelican/readers.py

@@ -367,7 +367,7 @@ class HTMLReader(BaseReader):
     class _HTMLParser(HTMLParser):
         def __init__(self, settings, filename):
             try:
-                # Python 3.4+
+                # Python 3.5+
                 HTMLParser.__init__(self, convert_charrefs=False)
             except TypeError:
                 HTMLParser.__init__(self)

pelican/tools/pelican_import.py

@@ -22,7 +22,7 @@ from pelican.settings import read_settings
 from pelican.utils import SafeDatetime, slugify
 
 try:
-    from html import unescape  # py3.4+
+    from html import unescape  # py3.5+
 except ImportError:
     from six.moves.html_parser import HTMLParser
     unescape = HTMLParser().unescape