create text file with list of dates

[cbt87]

There is a list of dates in python.py, deaths.py, merge.py, and scatters.py that stipulate the months for each scatterplot. If we want to update the animated gif to include additional months, we currently have to manually update each list of dates separately. We should create a text file with the list of dates that each of the source files should point to so we only have to update the list of dates once.