Version 3.1.1

What's Changed

• Docs fixes by @cliffckerr in #1149
• Make copy compatible with None by @RomeshA in #1158
• Add diagnostics by @cliffckerr in #1155
• Fix Pregnancy.finalize() crash for sims shorter than 1 year by @daniel-klein in #1151
• rc3.1.1 by @RomeshA in #1145

Full Changelog: v3.1.0...v3.1.1