Prove that open maps of locales are closed under pullbacks

This is [Topology.Locale/pullback_open](https://github.com/JetBrains/arend-lib/blob/4ea97da483c5bb2e6a22c8d09c03becf17756c7e/src/Topology/Locale.ard#L1178). A proof of this theorem can be found in Handbook of Categorical Algebra: Volume 3, Francis Borceux, 1994 (Chapter 1), Theorem 1.6.4.

Open src/Topology/Locale.ard at Topology.Locale/pullback_open and read the surrounding locale definitions. Consult Chapter 1, Theorem 1.6.4 of Handbook of Categorical Algebra, Volume 3, to guide the proof. Done means the theorem is proved in the cited file.

Written by the indexing model from the issue text.