update makefile

* `pipenv lock -r` is no more, it is now `pipenv requirements`
* had to recereate the virtualenv on 3.7
* don't have SAM installed anymore, now requires a binary download and
  installer (not pip/brew)

should really move this to CDK & GH Actions

Makefile

@@ -2,7 +2,7 @@ Pipfile.lock: Pipfile
 	pipenv lock 
 
 requirements.txt: Pipfile.lock
-	pipenv lock -r > requirements.txt
+	pipenv requirements > requirements.txt
 
 packaged.yml: template.yml requirements.txt *.py
 	pipenv run cfn-lint template.yml