Add partialmethod, fixes #1519

Returns correct method signature but test/completion/stdlib.py fails

test/completion/stdlib.py

@@ -158,6 +158,23 @@ tup[0]
 #? float()
 tup[1]
 
+class X:
+    def function(self, a, b):
+        return a, b
+    a = functools.partialmethod(function, 0)
+    kw = functools.partialmethod(function, b=1.0)
+
+#? int()
+X().a('')[0]
+#? str()
+X().a('')[1]
+
+tup = X().kw(1)
+#? int()
+tup[0]
+#? float()
+tup[1]
+
 def my_decorator(f):
     @functools.wraps(f)
     def wrapper(*args, **kwds):

test/test_inference/test_signature.py

@@ -65,6 +65,19 @@ d = functools.partial()
 '''
 
 
+partialmethod_code = '''
+import functools
+
+class X:
+    def func(self, a, b, c):
+        pass
+    a = functools.partialmethod(func)
+    b = functools.partialmethod(func, 1)
+    c = functools.partialmethod(func, 1, c=2)
+    d = functools.partialmethod()
+'''
+
+
 @pytest.mark.parametrize(
     'code, expected', [
         ('def f(a, * args, x): pass\n f(', 'f(a, *args, x)'),
@@ -82,6 +95,11 @@ d = functools.partial()
         (partial_code + 'b(', 'func(b, c)'),
         (partial_code + 'c(', 'func(b)'),
         (partial_code + 'd(', None),
+
+        (partialmethod_code + 'X().a(', 'func(a, b, c)'),
+        (partialmethod_code + 'X().b(', 'func(b, c)'),
+        (partialmethod_code + 'X().c(', 'func(b)'),
+        (partialmethod_code + 'X().d(', None),
     ]
 )
 def test_tree_signature(Script, environment, code, expected):