Add W606 warning for async and await keywords in Python 3.7

From https://docs.python.org/3/whatsnew/3.6.html#new-keywords

> async and await are not recommended to be used as variable, class,
> function or module names. Introduced by PEP 492 in Python 3.5, they
> will become proper keywords in Python 3.7. Starting in Python 3.6, the
> use of async or await as names will generate a DeprecationWarning.

By adding a warning to pycodestyle.py these future warnings and syntax
errors can be caught during static code analysis.

The await expression tests were taken from PEP-492.

https://www.python.org/dev/peps/pep-0492/#id58

1 files changed, 45 insertions, 0 deletions

diff --git a/testsuite/W60.py b/testsuite/W60.py
index cbe267d..030bec5 100644
--- a/testsuite/W60.py
+++ b/testsuite/W60.py
@@ -29,3 +29,48 @@ regex = r'''
 \\.png$
 '''
 s = '\\'
+#: W606
+async = 42
+#: W606
+await = 42
+#: W606
+def async():
+    pass
+#: W606
+def await():
+    pass
+#: W606
+class async:
+    pass
+#: W606
+class await:
+    pass
+#: Okay
+async def read_data(db):
+    data = await db.fetch('SELECT ...')
+#: Okay
+if await fut:
+    pass
+if (await fut):
+    pass
+if await fut + 1:
+    pass
+if (await fut) + 1:
+    pass
+pair = await fut, 'spam'
+pair = (await fut), 'spam'
+with await fut, open():
+    pass
+with (await fut), open():
+    pass
+await foo()['spam'].baz()()
+return await coro()
+return (await coro())
+res = await coro() ** 2
+res = (await coro()) ** 2
+func(a1=await coro(), a2=0)
+func(a1=(await coro()), a2=0)
+await foo() + await bar()
+(await foo()) + (await bar())
+-await foo()
+-(await foo())