* Update reline-0.5.7

* Update irb-1.13.1

(https://github.com/ruby/irb/pull/555)

* Remove all Ruby 2.6 support

* Drop Ruby 2.6 specific testing conditions

* Only run Ruby 2.7+ on CI

* Bump Ruby requirement to 2.7+

https://github.com/ruby/irb/commit/3f714b616c

`IRB::Inspector#inspect_value` errors
(https://github.com/ruby/irb/pull/511)

**Before**

```
irb(main):001:0> c = Cat.new "foo"
(Object doesn't support #inspect)
=>
```

**After**

```
irb(main):001:0> c = Cat.new "foo"
An error occurred when inspecting the object: #<NoMethodError: undefined method `is_a?' for foo:Cat

      if obj.is_a?(String)
            ^^^^^^>
Result of Kernel#inspect: #<Cat:0x0000000109090d80 @name="foo">
=>
```

https://github.com/ruby/irb/commit/cef125850d

https://github.com/ruby/irb/commit/07fae94862

* Use colorable: argument as the only coloring control

* Centalize color controling logic at Color.colorable?

There are 2 requirements for coloring output:

1. It's supported on the platform
2. The user wants it: `IRB.conf[:USE_COLORIZE] == true`

Right now we check 1 and 2 separately whenever we colorize things.
But it's error-prone because while 1 is the default of `colorable`
parameter, 2 always need to manually checked. When 2 is overlooked, it
causes issues like https://github.com/ruby/irb/pull/362

And there's 0 case where we may want to colorize even when the user
disables it. So I think we should merge 2 into `Color.colorable?` so it
can be automatically picked up.

* Add tests for all inspect modes

* Simplify inspectors' coloring logic

* Replace use_colorize? with Color.colorable?

* Remove Context#use_colorize cause it's redundant

https://github.com/ruby/irb/commit/1c53023ac4

1. `require` can mislead Ruby to load system irb's files and cause
   constant redefined warnings as other code loads the same module/class
   from lib folder.
2. Most files already use `require_relative`.

https://github.com/ruby/irb/commit/848d339f2e

The incompatible interface is not helpful, again if you want to use it
as a standalone library, falling it back to PP.

Original PP.pp also ends with `out << "\n"`.

https://github.com/ruby/irb/commit/4c74c7d84c

* moved rescue clause to `#inspect_value` to catch all failures in inspectors
* test with all (currently five kind of) inspect modes
  - tweaked the input due to only `Marshal` can inspect(dump) a `BasicObject`

https://github.com/ruby/irb/commit/9d112fab8e

https://github.com/ruby/irb/commit/dffcdb5269