Fix an INTERNAL ERROR when creating cobertura output for namespace package (#20112)

Fixes #19843

This fixes an Internal Error where namespace packages were not supported
properly. We have to use `os.path.isdir(path)` instead of trying to
catch an `IsADirectoryError` exception because of a bug on Windows which
causes it to throw a `PermissionError` instead in [the relevant
situation](https://discuss.python.org/t/permissionerror-errno-13-permission-denied-python-2023/22360/8),
which makes `except IsADirectoryError` unreliable. (We also can't just
`except (IsADirectoryError, PermissionError)` because what if there is
an actual permission error?)

Author: wyattscarpenter

mypy/report.py

@@ -141,12 +141,9 @@ def should_skip_path(path: str) -> bool:
 
 def iterate_python_lines(path: str) -> Iterator[tuple[int, str]]:
     """Return an iterator over (line number, line text) from a Python file."""
-    try:
+    if not os.path.isdir(path):  # can happen with namespace packages
         with tokenize.open(path) as input_file:
             yield from enumerate(input_file, 1)
-    except IsADirectoryError:
-        # can happen with namespace packages
-        pass
 
 
 class FuncCounterVisitor(TraverserVisitor):

test-data/unit/reports.test

@@ -547,3 +547,37 @@ namespace_packages = True
 </table>
 </body>
 </html>
+
+[case testReportCoberturaCrashOnNamespacePackages]
+# cmd: mypy --cobertura-xml-report report -p folder
+-- Regression test for https://github.com/python/mypy/issues/19843
+[file folder/subfolder/something.py]
+-- This output is not important, but due to the way tests are run we need to check it.
+[outfile report/cobertura.xml]
+<coverage timestamp="$TIMESTAMP" version="$VERSION" line-rate="1.0" branch-rate="0" lines-covered="0" lines-valid="0">
+  <sources>
+    <source>$PWD</source>
+  </sources>
+  <packages>
+    <package complexity="1.0" name="folder" branch-rate="0" line-rate="1.0">
+      <classes>
+        <class complexity="1.0" filename="folder" name="folder" branch-rate="0" line-rate="1.0">
+          <methods/>
+          <lines/>
+        </class>
+        <class complexity="1.0" filename="folder/subfolder" name="subfolder" branch-rate="0" line-rate="1.0">
+          <methods/>
+          <lines/>
+        </class>
+      </classes>
+    </package>
+    <package complexity="1.0" name="folder.subfolder" branch-rate="0" line-rate="1.0">
+      <classes>
+        <class complexity="1.0" filename="folder/subfolder/something.py" name="something.py" branch-rate="0" line-rate="1.0">
+          <methods/>
+          <lines/>
+        </class>
+      </classes>
+    </package>
+  </packages>
+</coverage>