chore(deps-dev): bump black from 21.7b0 to 21.8b0

Bumps [black](https://github.com/psf/black) from 21.7b0 to 21.8b0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/commits)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements-dev.txt

@@ -4,8 +4,6 @@
 #
 #    pip-compile --allow-unsafe --output-file=requirements-dev.txt requirements-dev.in
 #
-appdirs==1.4.4
-    # via black
 astor==0.8.1
     # via flake8-simplify
 astpretty==2.1.0
@@ -15,9 +13,7 @@ astroid==2.7.2
 asttokens==2.0.5
     # via flake8-aaa
 astunparse==1.6.3
-    # via
-    #   -r docs/requirements.txt
-    #   pytkdocs
+    # via -r docs/requirements.txt
 attrs==21.2.0
     # via
     #   flake8-bugbear
@@ -37,12 +33,10 @@ beautifulsoup4==4.9.3
     # via
     #   -r docs/requirements.txt
     #   mkdocs-htmlproofer-plugin
-black==21.7b0
+black==21.8b0
     # via -r requirements-dev.in
 cached-property==1.5.2
-    # via
-    #   -r docs/requirements.txt
-    #   pytkdocs
+    # via -r docs/requirements.txt
 certifi==2021.5.30
     # via
     #   -r docs/requirements.txt
@@ -170,19 +164,7 @@ importlib-metadata==4.7.1
     # via
     #   -r docs/requirements.txt
     #   -r requirements.txt
-    #   click
-    #   flake8
-    #   flake8-comprehensions
-    #   flake8-simplify
-    #   flake8-typing-imports
-    #   markdown
     #   mkdocs
-    #   pep517
-    #   pluggy
-    #   pre-commit
-    #   pytest
-    #   stevedore
-    #   virtualenv
 iniconfig==1.1.1
     # via pytest
 isort==5.9.3
@@ -305,6 +287,7 @@ pip-tools==6.2.0
     # via -r requirements-dev.in
 platformdirs==2.2.0
     # via
+    #   black
     #   pylint
     #   virtualenv
 pluggy==0.13.1
@@ -431,22 +414,12 @@ tomli==1.2.1
     # via
     #   black
     #   pep517
-typed-ast==1.4.3
-    # via
-    #   astroid
-    #   black
-    #   flake8-annotations
-    #   mypy
 typing-extensions==3.10.0.0
     # via
     #   -r docs/requirements.txt
     #   -r requirements.txt
-    #   astroid
     #   black
-    #   gitpython
-    #   importlib-metadata
     #   mypy
-    #   pytkdocs
 urllib3==1.26.6
     # via
     #   -r docs/requirements.txt
@@ -474,7 +447,6 @@ zipp==3.5.0
     #   -r docs/requirements.txt
     #   -r requirements.txt
     #   importlib-metadata
-    #   pep517
 
 # The following packages are considered to be unsafe in a requirements file:
 pip==21.2.4