chore: add specific requirements for pip-tools (#136)

requirements/pip-tools.in

@@ -0,0 +1,5 @@
+# Just the dependencies to run pip-tools, mainly for the "upgrade" make target
+
+-c constraints.txt
+
+pip-tools                 # Contains pip-compile, used to generate pip requirements files

requirements/pip-tools.txt

@@ -0,0 +1,20 @@
+#
+# This file is autogenerated by pip-compile with python 3.8
+# To update, run:
+#
+#    pip-compile --output-file=pip-tools.txt pip-tools.in
+#
+click==8.0.3
+    # via pip-tools
+pep517==0.12.0
+    # via pip-tools
+pip-tools==6.5.1
+    # via -r pip-tools.in
+tomli==2.0.1
+    # via pep517
+wheel==0.37.1
+    # via pip-tools
+
+# The following packages are considered to be unsafe in a requirements file:
+# pip
+# setuptools