Set up Github Actions CI (#440)

* Adjust tests: update Pip URLs, update expected output to match newer code
* Test library: improve failure message to eliminate the illusion of extra leading space in "actual" output
* Use Bats 1.2.0 like Pyenv
* Install greadlink for old MacOS
* Delete duplicate test

test/test_helper.bash

@@ -74,8 +74,10 @@ assert_failure() {
 
 assert_equal() {
   if [ "$1" != "$2" ]; then
-    { echo "expected: $1"
-      echo "actual:   $2"
+    { echo "expected:"
+      echo "$1"
+      echo "actual:"
+      echo "$2"
     } | flunk
   fi
 }