Fix assertion rewriter crash if cwd changes mid-testing

Unfortunately we need to get a `py.path.local` object to perform the fnmatch
operation, it is different from the standard `fnmatch` module because it
implements its own custom logic. So we need to use `py.path` to perform
the fnmatch for backward compatibility reasons.

Ideally we should be able to use a "pure path" in `pathlib` terms (a path
not bound to the file system), but we don't have those in pylib.

Fix #3973

changelog/3973.bugfix.rst

@@ -0,0 +1 @@
+Fix crash of the assertion rewriter if a test changed the current working directory without restoring it afterwards.

src/_pytest/assertion/rewrite.py

@@ -199,7 +199,11 @@ class AssertionRewritingHook(object):
 
         # For matching the name it must be as if it was a filename.
         parts[-1] = parts[-1] + ".py"
+        try:
             fn_pypath = py.path.local(os.path.sep.join(parts))
+        except EnvironmentError:
+            return False
+
         for pat in self.fnpats:
             # if the pattern contains subdirectories ("tests/**.py" for example) we can't bail out based
             # on the name alone because we need to match against the full path

testing/test_assertrewrite.py

@@ -1232,3 +1232,27 @@ class TestEarlyRewriteBailout(object):
         hook.fnpats[:] = ["tests/**.py"]
         assert hook.find_module("file") is not None
         assert self.find_module_calls == ["file"]
+
+    @pytest.mark.skipif(
+        sys.platform.startswith("win32"), reason="cannot remove cwd on Windows"
+    )
+    def test_cwd_changed(self, testdir):
+        testdir.makepyfile(
+            **{
+                "test_bar.py": """
+                import os
+                import shutil
+                import tempfile
+
+                d = tempfile.mkdtemp()
+                os.chdir(d)
+                shutil.rmtree(d)
+            """,
+                "test_foo.py": """
+                def test():
+                    pass
+            """,
+            }
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines("* 1 passed in *")