Release 1.5.1

Added

• Add a CI workflow which verifies that the darker --help output in README.rst
  is up to date.
• Only run linters, security checks and package builds once in the CI build.
• Small simplification: It doesn't matter whether isort was run or not, only
  whether changes were made.
• Refactor Black and isort file exclusions into one data structure.

Fixed

• darker --revision=a..b . now works since the repository root is now always
  considered to have existed in all historical commits.
• Ignore linter lines which refer to non-Python files or files outside the common root
  of paths on the command line. Fixes a failure when Pylint notifies about obsolete
  options in .pylintrc.
• For linting Darker's own code base, require Pylint 2.6.0 or newer. This avoids the
  need to skip the obsolete bad-continuation check now removed from Pylint.
• Fix linter output parsing for full Windows paths which include a drive letter.
• Stricter rules for linter output parsing.