getpath.py: fix dirname

also, fix finding prefix when in a venv

Modules/getpath.c

@@ -89,6 +89,12 @@ getpath_basename(PyObject *Py_UNUSED(self), PyObject *args)
     }
     Py_ssize_t end = PyUnicode_GET_LENGTH(path);
     Py_ssize_t pos = PyUnicode_FindChar(path, SEP, 0, end, -1);
+#ifdef ALTSEP
+    if (pos < 0) {
+        // try using altsep
+        pos = PyUnicode_FindChar(path, ALTSEP, 0, end, -1);
+    }
+#endif
     if (pos < 0) {
         return Py_NewRef(path);
     }
@@ -105,6 +111,12 @@ getpath_dirname(PyObject *Py_UNUSED(self), PyObject *args)
     }
     Py_ssize_t end = PyUnicode_GET_LENGTH(path);
     Py_ssize_t pos = PyUnicode_FindChar(path, SEP, 0, end, -1);
+#ifdef ALTSEP
+    if (pos < 0) {
+        // try using altsep
+        pos = PyUnicode_FindChar(path, ALTSEP, 0, end, -1);
+    }
+#endif
     if (pos < 0) {
         return PyUnicode_FromStringAndSize(NULL, 0);
     }

Modules/getpath.py

@@ -574,6 +574,9 @@ def search_up(prefix, *landmarks, test=isfile):
     # First try to detect prefix by looking alongside our runtime library, if known
     if library and not prefix:
         library_dir = dirname(library)
+        if os_name == 'nt' and is_mingw:
+            # QUIRK: On Windows, mingw Python DLLs are in the bin directory
+            library_dir = joinpath(library_dir, '..')
         if ZIP_LANDMARK:
             if os_name == 'nt':
                 # QUIRK: Windows does not search up for ZIP file