Fix `KeyError: (1, 0, 0, 6)` in backend_ps on Windows

Using code from backend_pdf: https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/backends/backend_pdf.py#L1002
Fix `KeyError: (1, 0, 0, 6)` on Windows
Merge pull request #2088 from cgohlke/patch-4

Fix `KeyError: (1, 0, 0, 6)` in backend_ps on Windows