Provide reproducible build support for 'last generated' statement

[jayaddison]

This updates the base theme layout to respect the SOURCE_DATE_EPOCH reproducible build environment variable when it is present, improving support for reproducible documentation builds.

Appearance / text format:

Source-date-epoch timestamp

Local system timestamp

Resolves #23.

[CaselIT]

I'm guessing a release to pypi would be appreciated once this is merged, right?

[zzzeek]

is it being consumed from teh release? our own reqs likely point to the github

[CaselIT]

don't know but I think debian would likely prefer to package a tag / release instead of the main branch

[jayaddison]

I'm guessing a release to pypi would be appreciated once this is merged, right?

Probably, yep - I think it's feasible to package an update without them, but in terms of having more easily comprehensible/comparable package versions, yep, that'd be helpful for Debian.

[jayaddison]

(also: thank you for the code review)

[jayaddison]

Ek, I forgot the space before the %Z. One moment..

[zzzeek]

let's try it out

[zzzeek]

ok that's released, 1.5.0 (though SQLAlchemy's own doc reqs point to the github)

[jayaddison]

It's looking good to me so far, thanks again both. An automated reproducibility test of mako with zzzeeksphinx v1.5.0 produces a successful result with this change in place. I'll continue to check on that and sqlalchemy documentation builds over the next few weeks.