reproducible HTML docs: set the locale

Fixes variation in plotdaynight.png

Author: mr-c

doc/make.py

@@ -7,7 +7,7 @@
 import sys
 
 def html():
-    os.system('sphinx-build -b html -c source/ -d build/doctrees . build/html')
+    os.system('LC_ALL=C.UTF-8 sphinx-build -b html -c source/ -d build/doctrees . build/html')
 
 def latex():
     if sys.platform != 'win32':