Reorganize paperspec commands and pin dependency versions (#2)

Reviewed-on: #2

schemas/README.md

@@ -64,7 +64,7 @@ The `Registry` maps that name to the file's contents, so nothing is fetched over
 Never load a schema alone.
 
 Any draft 2020-12 validator works the same way: give it every schema under its `$id`, then validate against the one you need.
-`python3 tools/check.py` validates every fixture with `jsonschema` this way, and checks that every schema keeps to the keyword subset.
+`paperspec check` validates every fixture with `jsonschema` this way, and checks that every schema keeps to the keyword subset.
 
 ## What a schema does not check