Fix issue when black or flake8 are already installed on the system.
Force usage of custom virtualenv when a tool requires extra Python packages. For instance flake8 requires plugin flake8-import-order. Even if the proper version of flake8 is available in PATH, reinstall it anyway in the custom virtualenv because it is too complex to figure out if flake8-import-order is also available in the same Python environment than system flake8.
Improve regular expression to extract a version from text.
flake8-import-order. Even if the proper version of flake8 is available in PATH, reinstall it anyway in the custom virtualenv because it is too complex to figure out ifflake8-import-orderis also available in the same Python environment than system flake8.