[ruby/pp] Add context to bind_call and skip 2.7+ test

The bind_call definition here is added primarily to support
running the tests on JRuby 9.3, which only supports Ruby 2.6
features. The excluded test appears to depend on inspect behavior
that changed after Ruby 2.6.

With these two changes the test suite runs green on Ruby 2.6 and
JRuby 9.3.

https://github.com/ruby/pp/commit/892c612958

test/test_pp.rb

@@ -5,7 +5,7 @@ require 'delegate'
 require 'test/unit'
 require 'ruby2_keywords'
 
-# Define bind_call for Ruby 2.6 and earlier
+# Define bind_call for Ruby 2.6 and earlier, to allow testing on JRuby 9.3
 class UnboundMethod
   unless public_method_defined?(:bind_call)
     def bind_call(obj, *args, &block)
@@ -168,7 +168,7 @@ class PPCycleTest < Test::Unit::TestCase
     a << HasInspect.new(a)
     assert_equal("[<inspect:[...]>]\n", PP.pp(a, ''.dup))
     assert_equal("#{a.inspect}\n", PP.pp(a, ''.dup))
-  end
+  end unless RUBY_VERSION < "2.7" # temporary mask to test on JRuby 9.3 (2.6 equivalent)
 
   def test_share_nil
     begin