Move docs requirements to docs/

This makes the file easier to locate, by placing it next to the stuff
that it will affect.

.readthedocs.yml

@@ -7,4 +7,4 @@ sphinx:
 python:
   version: 3.8
   install:
-    - requirements: tools/requirements/docs.txt
+    - requirements: docs/requirements.txt

MANIFEST.in

@@ -10,6 +10,7 @@ recursive-include  src/pip/_vendor *LICENSE*
 recursive-include  src/pip/_vendor *COPYING*
 
 include docs/docutils.conf
+include docs/requirements.txt
 
 exclude .coveragerc
 exclude .mailmap

noxfile.py

@@ -24,7 +24,7 @@ LOCATIONS = {
     "protected-pip": "tools/tox_pip.py",
 }
 REQUIREMENTS = {
-    "docs": "tools/requirements/docs.txt",
+    "docs": "docs/requirements.txt",
     "tests": "tools/requirements/tests.txt",
     "common-wheels": "tools/requirements/tests-common_wheels.txt",
 }

tox.ini

@@ -48,7 +48,7 @@ setenv =
 
 [testenv:docs]
 # Don't skip install here since pip_sphinxext uses pip's internals.
-deps = -r{toxinidir}/tools/requirements/docs.txt
+deps = -r{toxinidir}/docs/requirements.txt
 basepython = python3
 commands =
     sphinx-build -W -j auto -d {envtmpdir}/doctrees/html -b html docs/html docs/build/html