Make the documentation reproducible when rebuilt (#652)

The Reproducible Builds project aims to arrange for rebuilding the same
source code in a sufficiently similar environment to produce the same
installable packages every time, as a way to discourage supply-chain
attacks by making it possible to verify that a particular installable
package was built from the claimed source code.

Previously, if meson-python was built twice, at least a year apart,
then its documentation would contain different copyright dates. The
SOURCE_DATE_EPOCH environment variable is used here to avoid that
difference: the intention is that environments that want to produce
reproducible packages will set SOURCE_DATE_EPOCH to some suitable fixed
date (perhaps the date of the most recent git commit) which is held
constant across rebuilds.

See the specification for SOURCE_DATE_EPOCH for more details:
https://reproducible-builds.org/docs/source-date-epoch/

[smcv: Added commit message]

Co-authored-by: Chris Lamb <lamby@debian.org>

docs/conf.py

@@ -16,15 +16,21 @@
 
 import datetime
 import os
+import time
 import sys
 sys.path.insert(0, os.path.abspath('..'))
 import mesonpy
 
+build_date = datetime.datetime.fromtimestamp(
+    int(os.environ.get('SOURCE_DATE_EPOCH', time.time())),
+    tz=datetime.timezone.utc,
+)
+
 
 # -- Project information -----------------------------------------------------
 
 project = 'meson-python'
-copyright = f'2021\N{EN DASH}{datetime.date.today().year} The meson-python developers'
+copyright = f'2021\N{EN DASH}{build_date.year} The meson-python developers'
 
 # The short X.Y version
 version = mesonpy.__version__