Add a regression test for #15979, and fix linecount-report for Windows

mypy/report.py

@@ -172,11 +172,10 @@ def on_file(
     ) -> None:
         # Count physical lines.  This assumes the file's encoding is a
         # superset of ASCII (or at least uses \n in its line endings).
-        try:
+        if not os.path.isdir(tree.path):  # can happen with namespace packages
             with open(tree.path, "rb") as f:
                 physical_lines = len(f.readlines())
-        except IsADirectoryError:
-            # can happen with namespace packages
+        else:
             physical_lines = 0
 
         func_counter = FuncCounterVisitor()

test-data/unit/reports.test

@@ -547,3 +547,10 @@ namespace_packages = True
 </table>
 </body>
 </html>
+
+[case testLinecountReportCrashOnNamespacePackages]
+# cmd: mypy --linecount-report report -p folder
+--Regression test for https://github.com/python/mypy/issues/15979
+[file folder/subfolder/something.py]
+class Something:
+    pass