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

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"
-        fn_pypath = py.path.local(os.path.sep.join(parts))
+        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