docs: specify sphinx 1.7.9 in readme
docs: specify sphinx 1.7.9 in readme

(cherry picked from commit 99614d98e1c88467cbb862580bb2fe7c6b94195a)

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql