[ci][docs] downdrage sphinx and fix LaTeX error in R-package tests (#2977)

* downdrage sphinx

* fix latex errors

* Revert "fix latex errors"

This reverts commit 3cdf64748e.

* skip checksum verification

.ci/test_r_package.sh

@@ -38,8 +38,8 @@ if [[ $OS_NAME == "macos" ]]; then
     brew install qpdf
     brew cask install basictex
     export PATH="/Library/TeX/texbin:$PATH"
-    sudo tlmgr update --self
-    sudo tlmgr install inconsolata helvetic
+    sudo tlmgr --verify-repo=none update --self
+    sudo tlmgr --verify-repo=none install inconsolata helvetic
 
     wget -q https://cran.r-project.org/bin/macosx/R-${R_MAC_VERSION}.pkg -O R.pkg
     sudo installer \

docs/requirements_base.txt

@@ -1,3 +1,3 @@
-sphinx
+sphinx < 3.0
 sphinx_rtd_theme >= 0.3
 mock; python_version < '3'