Better substitution resolution and duration support

• Adding the resolve method to resolve substitution keys in 1 config tree with another config tree (@borissmidt) [#266]
• Support serializing timedelta and relativedelta to string (hocon, json etc.) (@gabis-precog) [#263]
• Upgrade to GitHub-native Dependabot (@dependabot-preview) [#260]
• Process substitution overrides in order (@JettJones) [#257]
• Fix duration parsing in lists (@olii) [#255]
• Add support for Python 3.9 and fix deprecation warning (@olii) [#254]