Restore python3-tk as an apt requirement (quantumlib#1876)

Strilanc · CirqBot · commit 83adc07917e8 · 2019-07-25T17:24:35.000-07:00
- I needed to install this to get matplotlib working
diff --git a/apt-system-requirements.txt b/apt-system-requirements.txt
@@ -1,2 +1,3 @@
 texlive-latex-base
 latexmk
+python3-tk
