Set SRC_URI

I'd thought I'd be able to rely on PyPI to get the package, via
cygport's python.org cygclass, but apparently OfflineIMAP only
distribute the Python2 version that way.

offlineimap.cygport

@@ -7,7 +7,7 @@ DESCRIPTION='OfflineIMAP is software that downloads your email mailbox(es) as
 local Maildirs.  OflineIMAP will synchronize both sides via IMAP.'
 HOMEPAGE=https://www.offlineimap.org
 LICENSE=GPL-2.0-or-later
-SRC_URI= # TODO
+SRC_URI="https://github.com/OfflineIMAP/offlineimap3/archive/refs/tags/v${PV}.tar.gz"
 ARCH=noarch
 
 BUILD_REQUIRES='python3 python3-pip'