Force py3.8 in tox.ini

It allows leveraging on OpenStack Yoga's upper-contraints
without any exception.

Issue-ID: DOC-782

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I4310d25f08b2dd730e798ac9f30ee83f0dfa5774
diff --git a/docs/tox.ini b/docs/tox.ini
index 037606f..64af58b 100644
--- a/docs/tox.ini
+++ b/docs/tox.ini
@@ -4,10 +4,10 @@
 skipsdist = true
 
 [testenv:docs]
-basepython = python3
+basepython = python3.8
 deps =
     -r{toxinidir}/requirements-docs.txt
-    -chttps://git.onap.org/doc/plain/etc/upper-constraints.os.txt
+    -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt
 commands =
     sphinx-build -b html -n -d {envtmpdir}/doctrees ./ {toxinidir}/_build/html
@@ -18,11 +18,11 @@
     sh
 
 [testenv:docs-linkcheck]
-basepython = python3
+basepython = python3.8
 deps =
     -r{toxinidir}/requirements-docs.txt
-    -chttps://git.onap.org/doc/plain/etc/upper-constraints.os.txt
+    -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt
 commands = echo "Link Checking not enforced"
 #commands = sphinx-build -b linkcheck -d {envtmpdir}/doctrees ./ {toxinidir}/_build/linkcheck
-whitelist_externals = echo
\ No newline at end of file
+whitelist_externals = echo