Update pip from 21.1.3 to 21.2 (#293)

requirements_dev.txt

@@ -1,7 +1,7 @@
 bumpversion==0.6.0
 coverage==5.5
 flake8==3.9.2
-pip==21.1.3
+pip==21.2
 pytest-runner==5.3.1
 pytest==6.2.4
 Sphinx==4.1.1