Add minimal versions and licensing information for requirements

Requirements should clearly state their license as this aids
in resolving potential licensing conflicts much more easily.

In addition, these requirements should denote a minimal version
based on the features they need from these requirements.

Afterwards, these minimal versions can be extended with a
lower-constraints job which verifies that all minimal
versions results in a functional application.

requirements.txt

-numpy
+numpy >= 1.20.0 # BSD

tests/requirements.txt

-black
-build
-flake8
-pylint
-pytest
-pytest-cov
+black >= 22.0.0 # MIT
+build >= 0.8.0 # MIT
+flake8 >= 5.0.0 # MIT
+pylint >= 2.15.0 # GPLv2
+pytest >= 7.0.0 # MIT
+pytest-cov >= 3.0.0 # MIT