Fix parsing relative imports with '...' tokens

Fixed #585.

Author: JukkaL

mypy/parse.py

@@ -164,9 +164,9 @@ def parse_import_from(self) -> Node:
 
         # Build the list of beginning relative tokens.
         relative = 0
-        while self.current_str() == ".":
-            self.expect('.')
-            relative += 1
+        while self.current_str() in (".", "..."):
+            relative += len(self.current_str())
+            self.skip()
 
         # Parse qualified name to actually import from.
         if self.current_str() == "import":

mypy/test/data/parse.test

@@ -3169,3 +3169,15 @@ MypyFile:1(
         z))
     Block:1(
       PassStmt:1())))
+
+[case testRelativeImportWithEllipsis]
+from ... import x
+[out]
+MypyFile:1(
+  ImportFrom:1(..., [x : x]))
+
+[case testRelativeImportWithEllipsis2]
+from .... import x
+[out]
+MypyFile:1(
+  ImportFrom:1(...., [x : x]))