Update README.md for handling mitmproxy (#6019)

Update docs for handling mitmproxy in failure testing.

CONTRIBUTING.md

@@ -35,6 +35,12 @@ why we ask this as well as instructions for how to proceed, see the
   # Optionally, you might instead want to use `make install-all`
   # since `multi_extension` regression test would fail due to missing downgrade scripts.
   cd src/test/regress
+
+  pip install pipenv
+  pipenv --rm
+  pipenv install
+  pipenv shell
+
   make check
   ```
 
@@ -53,7 +59,7 @@ why we ask this as well as instructions for how to proceed, see the
                           autoconf flex git libcurl4-gnutls-dev libicu-dev \
                           libkrb5-dev liblz4-dev libpam0g-dev libreadline-dev \
                           libselinux1-dev libssl-dev libxslt1-dev libzstd-dev \
-                          make uuid-dev mitmproxy
+                          make uuid-dev
   ```
 
 2. Get, build, and test the code
@@ -67,6 +73,12 @@ why we ask this as well as instructions for how to proceed, see the
   # Optionally, you might instead want to use `sudo make install-all`
   # since `multi_extension` regression test would fail due to missing downgrade scripts.
   cd src/test/regress
+
+  pip install pipenv
+  pipenv --rm
+  pipenv install
+  pipenv shell
+
   make check
   ```
 
@@ -111,6 +123,12 @@ why we ask this as well as instructions for how to proceed, see the
   # Optionally, you might instead want to use `sudo make install-all`
   # since `multi_extension` regression test would fail due to missing downgrade scripts.
   cd src/test/regress
+
+  pip install pipenv
+  pipenv --rm
+  pipenv install
+  pipenv shell
+
   make check
   ```
 

src/test/regress/mitmscripts/README.md

@@ -17,9 +17,18 @@ Automated Failure Testing works by inserting a network proxy (mitmproxy) between
 
 ## Getting Started
 
-First off, to use this you'll need mitmproxy, I recommend version `7.0.4`, and I also recommend running it with `python 3.9`. This script integrates pretty deeply with mitmproxy so other versions might fail to work.
+First off, to use this you'll need mitmproxy.
+Currently, we rely on a [fork](https://github.com/thanodnl/mitmproxy/tree/fix/tcp-flow-kill) to run the failure tests.
+We recommned using pipenv to setup your failure testing environment since that will handle installing the fork
+and other dependencies which may be updated/changed.
 
-I highly recommend using pipenv to install mitmproxy. It lets you easily manage isolated environments (instead of installing python packages globally). If you've heard of virtualenv, pipenv is that but much easier to use.
+Setting up pipenv is easy if you already have python and pip set up:
+```bash
+pip install pipenv
+```
+
+If the Pipfile requires a version you do not have, simply install that python version and retry.
+Pipenv should be able to find the newly installed python and set up the environment.
 
 Once you've installed it: