Bump puma from 3.12.6 to 4.3.9

Bumps [puma](https://github.com/puma/puma) from 3.12.6 to 4.3.9.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.6...v4.3.9)

---
updated-dependencies:
- dependency-name: puma
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -46,10 +46,12 @@ GEM
     minitest (5.11.3)
     multipart-post (2.0.0)
     mustermann (1.0.3)
+    nio4r (2.5.8)
     pry (0.11.3)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
-    puma (3.12.6)
+    puma (4.3.9)
+      nio4r (~> 2.0)
     rack (2.0.8)
     rack-protection (2.0.3)
       rack