Use the platform.python_implementation because it's the most compat

See <pypa/packaging#72 (comment)>
for a discussion of the various setuptools different aliases
support. This means we don't work with 20.2 to 20.6 (released
mid-Feb till late-Mar 2016).

requirements-optional.txt

@@ -10,11 +10,11 @@ charade
 
 # lxml is supported with its own treebuilder ("lxml") and otherwise
 # uses the standard ElementTree support
-lxml ; platform_python_implementation == 'CPython'
+lxml ; platform.python_implementation == 'CPython'
 
 # DATrie can be used in place of our Python trie implementation for
 # slightly better parsing performance.
-datrie ; platform_python_implementation == 'CPython'
+datrie ; platform.python_implementation == 'CPython'
 
 # Can be used to force attributes to be serialized in alphabetical
 # order.

setup.py

@@ -65,8 +65,8 @@
 
           # A conditional extra will only install these items when the extra is
           # requested and the condition matches.
-          "datrie:platform_python_implementation == 'CPython'": ["datrie"],
-          "lxml:platform_python_implementation == 'CPython'": ["lxml"],
+          "datrie:platform.python_implementation == 'CPython'": ["datrie"],
+          "lxml:platform.python_implementation == 'CPython'": ["lxml"],
 
           # Standard extras, will be installed when the extra is requested.
           "genshi": ["genshi"],
@@ -77,6 +77,6 @@
           # extra that will be installed whenever the condition matches and the
           # all extra is requested.
           "all": ["genshi", "charade"],
-          "all:platform_python_implementation == 'CPython'": ["datrie", "lxml"],
+          "all:platform.python_implementation == 'CPython'": ["datrie", "lxml"],
       },
       )