Handle python 3.6 f-strings without error

Author: Adrien Chauve

mypy/fastparse.py

@@ -769,6 +769,11 @@ def visit_Str(self, n: ast35.Str) -> Union[UnicodeExpr, StrExpr]:
         else:
             return UnicodeExpr(n.s)
 
+    # JoinedStr(expr* values)
+    @with_line
+    def visit_JoinedStr(self, n: ast35.JoinedStr) -> StrExpr:
+        return StrExpr(n.values[0])
+
     # Bytes(bytes s)
     @with_line
     def visit_Bytes(self, n: ast35.Bytes) -> Union[BytesExpr, StrExpr]:

test-data/unit/check-expressions.test

@@ -1157,6 +1157,15 @@ b'%c' % (123)
 [case testUnicodeInterpolation_python2]
 u'%s' % (u'abc',)
 
+
+-- F-String
+-- --------
+
+
+[case testFStringParseOk]
+a = f'foobar'
+
+
 -- Lambdas
 -- -------