Update tbump requirement from ~=6.3.2 to ~=6.8.0 (#1493)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements_test.txt

@@ -6,6 +6,6 @@ coverage~=5.5
 pre-commit~=2.19
 pytest-cov~=3.0
 contributors-txt>=0.7.3
-tbump~=6.3.2
+tbump~=6.8.0
 types-typed-ast; implementation_name=="cpython" and python_version<"3.8"
 types-pkg_resources==0.1.3