Merge pull request jazzband#1405 from nicoa/1403_fix_failing_tox_tests

tox.ini

@@ -11,12 +11,16 @@ extras =
     testing
     coverage: coverage
 deps =
+    # due to broken CI, to be removed as soon as it is fixed, compare https://github.com/pypa/setuptools/issues/2687
+    setuptools<57
     pipprevious: pip==20.3.*
     piplatest: pip
     pipmain: -e git+https://github.com/pypa/pip.git@main#egg=pip
     pip20.3: pip==20.3.*
 setenv =
     coverage: PYTEST_ADDOPTS=--strict --doctest-modules --cov --cov-report=term-missing --cov-report=xml {env:PYTEST_ADDOPTS:}
+    pipprevious,pip20.3: PIP_DISABLE_PIP_VERSION_CHECK=1
+    piplatest: PIP_USE_FEATURE=in-tree-build
 commands_pre =
     piplatest: python -m pip install -U pip
     pip --version