[ci] Use Ubuntu Focal image at Travis (#3393)

* Use Ubuntu Focal image at Travis

* test gcc

* test Bionic and gcc

* test Focal with another Python version for GPU test

* revert testing changes

* install linkchecker via pip

* simplify linkchecker config

.ci/test.sh

@@ -16,7 +16,7 @@ cd $BUILD_DIRECTORY
 if [[ $TRAVIS == "true" ]] && [[ $TASK == "check-docs" ]]; then
     cd $BUILD_DIRECTORY/docs
     conda install -q -y -n $CONDA_ENV -c conda-forge doxygen
-    pip install --user -r requirements.txt rstcheck
+    pip install --user -r requirements.txt rstcheck git+git://github.com/linkchecker/linkchecker.git@b9390c9ef63f7e1e210b48bc7fe97f76e8d39501
     # check reStructuredText formatting
     cd $BUILD_DIRECTORY/python-package
     rstcheck --report warning `find . -type f -name "*.rst"` || exit -1
@@ -26,11 +26,7 @@ if [[ $TRAVIS == "true" ]] && [[ $TASK == "check-docs" ]]; then
     make html || exit -1
     find ./_build/html/ -type f -name '*.html' -exec \
     sed -i'.bak' -e 's;\(\.\/[^.]*\.\)rst\([^[:space:]]*\);\1html\2;g' {} \;  # emulate js function
-    if [[ $OS_NAME == "linux" ]]; then
-        sudo apt-get update
-        sudo apt-get install linkchecker
     linkchecker --config=.linkcheckerrc ./_build/html/*.html || exit -1
-    fi
     # check the consistency of parameters' descriptions and other stuff
     cp $BUILD_DIRECTORY/docs/Parameters.rst $BUILD_DIRECTORY/docs/Parameters-backup.rst
     cp $BUILD_DIRECTORY/src/io/config_auto.cpp $BUILD_DIRECTORY/src/io/config_auto-backup.cpp

.travis.yml

@@ -9,7 +9,7 @@ git:
 os:
   - linux
   - osx
-dist: bionic
+dist: focal
 osx_image: xcode12
 
 env:

docs/.linkcheckerrc

@@ -6,7 +6,6 @@ sslverify=0
 [filtering]
 ignore=
   pythonapi/lightgbm\..*\.html.*
-  http://sphinx-doc.org
 ignorewarnings=http-robots-denied,https-certificate-error
 checkextern=1