Let's not go to 18+

What's Changed

• Drop Python3.7 support - heavily deprecated upstream. by @JSKenyon in #268
• Escape messages before emitting. by @JSKenyon in #267
• Add recursive finder for filelike objects by @JSKenyon in #266
• Attempt at making substitutions work inside dicts and lists. by @JSKenyon in #269
• fixes #273. Adds schema flattening to clickify_parameters by @o-smirnov in #274
• Honour always tag even when step selection is in use. by @JSKenyon in #272
• fixes #275 by @o-smirnov in #276

Full Changelog: 2.0rc17...2.0rc18