docs: improve docs on running tests locally (#14102)

* Fix missing items in ToC

Thanks to whatever autoformatting tool was running (:

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* Fix minimal Python version required

Currently Python 3.7 is still supported by Superset — cf.
4b23d0ecca/setup.py (L160)

I assue it is thus safe to run the test with Py3.7.

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* List MySQL/MariaDB as OS dependecy for test

As raised in issue #14052, MySQL (or MariaDB) is required for being able
to install the Python requirements.
(I assume due to `mysqlclient` dependency — cf.
https://github.com/apache/superset/blob/master/requirements/development.txt#L23)

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* Fix which dependencies are installed for tests

To install `tox` (for running the test suite), we want to install the…
testing dependencies, rather than “simply” the local ones.

Note that the `requirements/testing` is a superset of the
`requirements/local`:
https://github.com/apache/superset/blob/master/requirements/testing.txt
vs https://github.com/apache/superset/blob/master/requirements/local.txt

Linked to issue #14052

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* Emphasize how to name demo admin user

Related to #14052

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* Fix max Python version required

Indeed, Python 3.9 isn't yet supported (cf. e.g., issue #14159).

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

CONTRIBUTING.md

@@ -77,6 +77,8 @@ little bit helps, and credit will always be given.
     - [Python Testing](#python-testing)
     - [Frontend Testing](#frontend-testing)
     - [Integration Testing](#integration-testing)
+    - [Debugging Server App](#debugging-server-app)
+    - [Debugging Server App in Kubernetes Environment](#debugging-server-app-in-kubernetes-environment)
     - [Storybook](#storybook)
   - [Translating](#translating)
     - [Enabling language selection](#enabling-language-selection)
@@ -418,9 +420,10 @@ For example, the image referenced above actually lives in `superset-frontend/ima
 
 #### OS Dependencies
 
-Make sure your machine meets the [OS dependencies](https://superset.apache.org/docs/installation/installing-superset-from-scratch#os-dependencies) before following these steps.
+Make sure your machine meets the [OS dependencies](https://superset.apache.org/docs/installation/installing-superset-from-scratch#os-dependencies) before following these steps.  
+You also need to install MySQL or [MariaDB](https://mariadb.com/downloads).
 
-Ensure Python versions >3.7, Then proceed with:
+Ensure that you are using Python version 3.7 or 3.8, then proceed with:
 
 ````bash
 # Create a virtual environment and activate it (recommended)
@@ -428,12 +431,12 @@ python3 -m venv venv # setup a python3 virtualenv
 source venv/bin/activate
 
 # Install external dependencies
-pip install -r requirements/local.txt
+pip install -r requirements/testing.txt
 
 # Install Superset in editable (development) mode
 pip install -e .
 
-# Create an admin user in your metadata database
+# Create an admin user in your metadata database (use `admin` as username to be able to load the examples)
 superset fab create-admin
 
 # Initialize the database
@@ -442,11 +445,12 @@ superset db upgrade
 # Create default roles and permissions
 superset init
 
-# Load some data to play with (you must create an Admin user with the username `admin` for this command to work)
+# Load some data to play with.
+# Note: you MUST have previously created an admin user with the username `admin` for this command to work.
 superset load-examples
 
 # Start the Flask dev web server from inside your virtualenv.
-# Note that your page may not have css at this point.
+# Note that your page may not have CSS at this point.
 # See instructions below how to build the front-end assets.
 FLASK_ENV=development superset run -p 8088 --with-threads --reload --debugger