Adding new features in tox.ini (#1457)

The scripts/fmt.sh script has been replaced with a new tox command: tox -e fmt.
It behaves exactly the same way, except that it's executed in a venv.

Another new command, tox -e style, also checks for formatting mistakes.
It returns a code error, if an error has been found in one of the files.
It does not edit any files. It would be suitable for a CI.

The file Makefile has been modified so that "make format" still works.

Makefile

@@ -44,4 +44,4 @@ test:
 # Format code
 .PHONY: format
 format:
-	./scripts/fmt.sh
+	tox -e fmt

tox.ini

@@ -6,4 +6,28 @@ usedevelop = True
 deps =
     -rrequirements.txt
     pytest
-commands = pytest tests
+commands = pytest tests
+
+[testenv:fmt]
+allowlist_externals =
+    bash
+deps =
+    black
+    autoflake
+    pyupgrade
+    isort
+commands =
+    bash -c 'find tuxemon/ -iname "*.py" -type f | parallel pyupgrade --exit-zero-even-if-changed --py38-plus --keep-runtime-typing'
+    autoflake -r -i --imports=tuxemon,pygame --ignore-init-module-imports tuxemon/
+    isort tuxemon tests
+    black tuxemon tests
+
+[testenv:style]
+deps =
+    black
+    autoflake
+    isort
+commands =
+    autoflake -r -i --imports=tuxemon,pygame --ignore-init-module-imports --check-diff --quiet tuxemon/
+    isort --check --diff --quiet tuxemon tests
+    black --check --diff --quiet tuxemon tests