Bug 1661783 - Re-enable test_rst.py on Linux.r=rstewart

Differential Revision: https://phabricator.services.mozilla.com/D89311

tools/lint/test/python.ini

@@ -17,7 +17,7 @@ skip-if = os == "win"
 [test_shellcheck.py]
 [test_rst.py]
 requirements = tools/lint/rst/requirements.txt
-skip-if = os == "mac" || os == "linux"  # pip unable to install
+skip-if = os == "mac" # pip unable to install
 [test_codespell.py]
 skip-if = os == "win" || os == "mac"  # codespell installed on Linux
 [test_yaml.py]