Backport PR pandas-dev#38841: Update conf.py to execute imports during pdf building

Author: gepcel

doc/source/conf.py

@@ -427,7 +427,7 @@
 
 
 ipython_warning_is_error = False
-ipython_exec_lines = [
+ipython_execlines = [
     "import numpy as np",
     "import pandas as pd",
     # This ensures correct rendering on system with console encoding != utf8