remove tox<4 constraint in build.yml

tox.ini has been updated in main repo.

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Install base dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install 'tox<4'
+          python -m pip install tox
       - name: Install Gammapy
         run: |
           git clone https://github.com/gammapy/gammapy.git