Bugfix/fix failing tests on windows due to relpath (#8094)

Author: Netzeband

README.md

@@ -221,7 +221,7 @@ This submodule contains types for the Python standard library.
 
 Due to the way git submodules work, you'll have to do
 ```
-  git submodule update typeshed
+  git submodule update mypy/typeshed
 ```
 whenever you change branches, merge, rebase, or pull.
 

mypy/report.py

@@ -459,7 +459,12 @@ def on_file(self,
                 type_map: Dict[Expression, Type],
                 options: Options) -> None:
         self.last_xml = None
-        path = os.path.relpath(tree.path)
+
+        try:
+            path = os.path.relpath(tree.path)
+        except ValueError:
+            return
+
         if should_skip_path(path):
             return
 
@@ -813,7 +818,12 @@ def on_file(self,
                 modules: Dict[str, MypyFile],
                 type_map: Dict[Expression, Type],
                 options: Options) -> None:
-        path = os.path.relpath(tree.path)
+
+        try:
+            path = os.path.relpath(tree.path)
+        except ValueError:
+            return
+
         if should_skip_path(path):
             return
 

test-data/unit/check-incremental.test

@@ -4117,7 +4117,9 @@ class C:
 [out2]
 main:5: error: Incompatible types in assignment (expression has type "str", variable has type "int")
 
-[case testBazelFlagIgnoresFileChanges]
+[case testBazelFlagIgnoresFileChanges-skip]
+-- This test fails on windows, when the mypy source in on a different drive than
+--   the run-directory. In this case os.path.relpath(...) fails with an exception
 -- Since the initial run wrote a cache file, the second run ignores the source
 # flags: --bazel
 from a import f