Pin macOS installer Sphinx to v2.0.1 (GH-13774)

diff --git a/Mac/BuildScript/build-installer.py b/Mac/BuildScript/build-installer.py
index 60e28fe..2c606b9 100755
--- a/Mac/BuildScript/build-installer.py
+++ b/Mac/BuildScript/build-installer.py
@@ -1058,6 +1058,7 @@
     runCommand('make clean')
     # Create virtual environment for docs builds with blurb and sphinx
     runCommand('make venv')
+    runCommand('venv/bin/python3 -m pip install -U Sphinx==2.0.1')
     runCommand('make html PYTHON=venv/bin/python')
     os.chdir(curDir)
     if not os.path.exists(docdir):