docs: update read-the-docs version of sphinx

This is specified both as best-practice to specify the defaults
explicitly, and as required for updating sphinx from v1.8.5

Refs: https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
PR-URL: https://github.com/libuv/libuv/pull/3154
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

.readthedocs.yaml

@@ -0,0 +1,11 @@
+version: 2
+
+sphinx:
+  builder: html
+  configuration: null
+  fail_on_warning: false
+
+python:
+  version: 3.8
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,42 @@
+# primary
+Sphinx==3.5.4
+
+# dependencies
+alabaster==0.7.12
+appdirs==1.4.3
+Babel==2.9.0
+CacheControl==0.12.6
+certifi==2019.11.28
+chardet==3.0.4
+colorama==0.4.3
+contextlib2==0.6.0
+distlib==0.3.0
+distro==1.4.0
+docutils==0.16
+html5lib==1.0.1
+idna==2.8
+imagesize==1.2.0
+ipaddr==2.2.0
+Jinja2==2.11.3
+lockfile==0.12.2
+MarkupSafe==1.1.1
+msgpack==0.6.2
+packaging==20.3
+pep517==0.8.2
+progress==1.5
+Pygments==2.8.1
+pyparsing==2.4.6
+pytoml==0.1.21
+pytz==2021.1
+requests==2.22.0
+retrying==1.3.3
+six==1.14.0
+snowballstemmer==2.1.0
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.4
+urllib3==1.25.8
+webencodings==0.5.1