Fix for works with old and new versions

Author: mokamoto12

lib/poetry.js

@@ -35,7 +35,11 @@ function pyprojectTomlToRequirements() {
 
   const editableFlag = new RegExp(/^-e /gm);
   const sourceRequirements = path.join(this.servicePath, 'requirements.txt');
-  const requirementsContents = res.stdout.toString();
+  const requirementsContents =
+    res.stdout.toString() ||
+    fse.readFileSync(sourceRequirements, {
+      encoding: 'utf-8'
+    });
 
   if (requirementsContents.match(editableFlag)) {
     this.serverless.cli.log(