pygments_dark_style is always being reported as a configuration change

[ncoghlan]

Working on venvstacks docs today, and I noticed that every incremental docs build was generating this config change notice:

loading pickled environment... The configuration has changed (1 option: 'pygments_dark_style')

At first I thought it was because the pygments themes weren't being explicitly set, but adding the following didn't resulting in any change to the behaviour:

pygments_style = "sphinx"
pygments_dark_style = "monokai"

(and I've confirmed that monokai is available in the docs build environment)