Upgrade klayout to 0.26.8.

Summary:
Upgrade klayout to 0.26.8.

Changelog:

• Enhancement: A new method is available which enables copying of multiple cells into a target layout while maintaining common subcells are single cells.
• Bugfix: Points have not been selected properly when inside mirrored subcells in partial edit mode.
• Bugfix: Qt5 QTimer timeout signal was not available to Python or Ruby. This also applied to all "private" signals of Qt5. QTimer is just one important example for this case.
• Bugfix: A problem with pseudo-const Python/Ruby objects in the netlist API was fixed.
• Bugfix: Basic authentication for package manager now uses UTF-8 encoding. Most servers expect this kind of encoding today. UTF password characters are supported this way.

Test Plan:

As before, open a few GDS files (kB to GB in size) and browse the layout. Open Help->About and check it ruby/python bindings are present in the `Build options'.

    - Open a few GDS files (kB to GB in size) and browse the layout.
    - Open Help->About and check it ruby/python bindings are present in the `Build options` and ~20 .so files are present in `Binary Extensions`.
    - Start python3, import `klayout` and `pya` modules.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D9724