IVGCVSW-7473 Update python version to 3.8 to build pyarmnn in Ubuntu 20.04

Signed-off-by: Nikhil Raj <nikhil.raj@arm.com>
Change-Id: Ied41eda5c1979d9a2dae192b437fbeb688a0fd13
diff --git a/python/pyarmnn/tox.ini b/python/pyarmnn/tox.ini
index 652641c..f17db7d 100644
--- a/python/pyarmnn/tox.ini
+++ b/python/pyarmnn/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 skip_missing_interpreters=true
-envlist = py36, py37
+envlist = py38
 
 [testenv]
 deps = pytest==5.2.0
@@ -20,7 +20,7 @@
 
 [testenv:devenv]
 envdir = env
-basepython = python3.6
+basepython = python3.8
 usedevelop = True
 deps = {[testenv]deps}
        tox
@@ -28,7 +28,7 @@
 commands = python -c "import sys; print('Dev environment created: ' + sys.executable)"
 
 [testenv:gen]
-basepython = python3.6
+basepython = python3.8
 skip_install = True
 usedevelop = True
 passenv =
@@ -40,7 +40,7 @@
     python setup.py build_ext --inplace
 
 [testenv:doc]
-basepython = python3.6
+basepython = python3.8
 deps = pdoc3==0.6.3
 passenv =
     PYARMNN_DEV_VER
@@ -48,7 +48,7 @@
     python ./scripts/generate_docs.py --html --output-dir docs pyarmnn --force --template-dir=./docs_conf
 
 [testenv:pylint]
-basepython = python3.6
+basepython = python3.8
 deps = pylint==2.3.1
        numpy==1.17.2
 recreate = False