pragmatismatic: run flaky tests three times to see if we can get more "green" CI runs this way ...

thanks to @the-compiler also modernize plugin usage

python/README.rst

@@ -65,7 +65,7 @@ Afterwards ``which python`` tells you that it comes out of the "venv"
 directory that contains all python install artifacts. Let's first
 install test tools::
 
-    pip install pytest pytest-timeout requests
+    pip install pytest pytest-timeout pytest-rerunfailures requests
 
 then cargo-build and install the deltachat bindings::