Update all dependencies (#104)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

requirements_dev.txt

@@ -1,9 +1,9 @@
 bump2version==1.0.1
 coverage==5.5
-flake8==3.8.4
-pip==21.0.1
-pytest==6.2.2
-Sphinx==3.5.1
-tox==3.22.0
-twine==3.3.0
+flake8==3.9.2
+pip==21.1.3
+pytest==6.2.4
+Sphinx==3.5.4
+tox==3.23.1
+twine==3.4.1
 wheel==0.36.2