Prepare CHANGES.md for release 21.8b0 (#2458)

Hopefully my first release doesn't end up in flames 🔥

Commit history before merge:

* Prepare CHANGES.md for release 21.8b0
* I need to add a check for this too.

CHANGES.md

@@ -1,6 +1,6 @@
 # Change Log
 
-## Unreleased
+## 21.8b0
 
 ### _Black_
 
@@ -17,7 +17,8 @@
 
 ### _Blackd_
 
-- Replace sys.exit(-1) with raise ImportError (#2440)
+- Replace sys.exit(-1) with raise ImportError as it plays more nicely with tools that
+  scan installed packages (#2440)
 
 ### Integrations
 

docs/integrations/source_version_control.md

@@ -7,7 +7,7 @@ Use [pre-commit](https://pre-commit.com/). Once you
 ```yaml
 repos:
   - repo: https://github.com/psf/black
-    rev: 21.7b0
+    rev: 21.8b0
     hooks:
       - id: black
         language_version: python3 # Should be a command that runs python3.6+

docs/usage_and_configuration/the_basics.md

@@ -173,13 +173,13 @@ You can check the version of _Black_ you have installed using the `--version` fl
 
 ```console
 $ black --version
-black, version 21.5b0
+black, version 21.8b0
 ```
 
 An option to require a specific version to be running is also provided.
 
 ```console
-$ black --required-version 21.5b2 -c "format = 'this'"
+$ black --required-version 21.8b0 -c "format = 'this'"
 format = "this"
 $ black --required-version 31.5b2 -c "still = 'beta?!'"
 Oh no! 💥 💔 💥 The required version does not match the running version!