jailkit: Use Python 3.8 instead of system Python 2

- This formula has a relatively low number of installs (145 on macOS and
  1 on Linux over the past 365 days), but it also didn't require very
  many hacks to make it work with Python 3.8, so I'm going for it.
- It doesn't have a test, so we won't know if it _actually_ works, but I
  don't know the software enough to write a test for it now.

Closes #50470.

Signed-off-by: Alexander Bayandin <a.bayandin@gmail.com>

Formula/jailkit.rb

@@ -10,7 +10,11 @@ class Jailkit < Formula
     sha256 "25e55b2cc8572ad043c97ad2b86e08f93ef1a69c6fb66d1bf1630c4c5092bdfc" => :high_sierra
   end
 
+  depends_on "python@3.8"
+
   def install
+    ENV["PYTHONINTERPRETER"] = Formula["python@3.8"].opt_bin/"python3"
+
     system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
     system "make", "install"
   end