I have Ansible tower with 3.5 on RHEL 7.6 with the following setup:
# python -V
Python 2.7.5
# ansible --version
ansible 2.7.6
config file = /etc/ansible/ansible.cfg
configured module search path = [u'/root/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
ansible python module location = /usr/lib/python2.7/site-packages/ansible
executable location = /usr/bin/ansible
python version = 2.7.5 (default, Jun 11 2019, 14:33:56) [GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]

# pip freeze
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Ple ase upgrade your Python as Python 2.7 won't be maintained after that date. A fut ure version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/relea se-process/#python-2-support
ansible==2.7.6
Babel==0.9.6
backports.ssl-match-hostname==3.5.0.1
cffi==1.6.0
chardet==2.2.1
configobj==4.7.2
cryptography==1.7.2
decorator==3.4.0
dnspython==1.12.0
enum34==1.0.4
ethtool==0.8
futures==3.1.1
httplib2==0.9.2
idna==2.4
iniparse==0.4
insights-client==3.0.6
ipaddr==2.1.11
ipaddress==1.0.16
javapackages==1.0.0
Jinja2==2.8
jmespath==0.9.0
kitchen==1.1.1
lxml==3.2.1
M2Crypto==0.21.1
Magic-file-extensions==0.2
MarkupSafe==0.11
meld3==0.6.10
mercurial==2.6.2
paramiko==2.1.1
pciutils==1.7.3
perf==0.1
ply==3.4
psycopg2==2.7.5
pyasn1==0.1.9
pycparser==2.14
pycrypto==2.6.1
pycurl==7.19.0
pygobject==3.22.0
pygpgme==0.3
pyinotify==0.9.4
pyliblzma==0.5.3
pyOpenSSL==0.13.1
python-dateutil==1.5
python-dmidecode==3.10.13
python-keyczar==0.71rc0
python-linux-procfs==0.4.9
python-qpid-proton==0.24.0
pyudev==0.15
pyxattr==0.5.1
PyYAML==3.10
requests==2.6.0
rhnlib==2.5.65
schedutils==0.4
six==1.9.0
slip==0.4.0
slip.dbus==0.4.0
subscription-manager==1.24.13
supervisor==3.1.4
syspurpose==1.24.13
urlgrabber==3.10
urllib3==1.10.2
virtualenv==15.1.0
yum-metadata-parser==1.1.4

And I have created a virtual environment as follow:
# yum install gcc openssl-devel libffi-devel python-devel
# yum install python27
# scl enable python27 bash
# umask 0022
# mkdir /opt/new/custom-venv
# chmod 0755 /opt/new/
# virtualenv /opt/new/custom-venv --python=python2.7
# source /opt/new/custom-venv/bin/activate
(custom-venv)# /opt/new/custom-venv/bin/pip setuptools --upgrade
(custom-venv)# /opt/new/custom-venv/bin/pip install --upgrade pip
(custom-venv)# /opt/new/custom-venv/bin/pip install python-memcached psutil ansible==2.8.4
(custom-venv)# /opt/new/custom-venv/bin/pip ovirt-engine-sdk-python==4.3.0
(custom-venv)# /opt/new/custom-venv/bin/pip install hpICsp==1.0.2
(custom-venv)# /opt/new/custom-venv/bin/pip install hpOneView==4.8.0
(custom-venv)# rsync -avh --progress /tmp/oneview/library /opt/new/custom-venv/lib/python2.7/site-packages/ansible/oneview-ansible
(custom-venv)# pip freeze
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
ansible==2.8.4
cffi==1.13.2
cryptography==2.8
enum34==1.1.6
future==0.18.2
hpICsp==1.0.2
hpOneView==4.8.0
ipaddress==1.0.23
Jinja2==2.10.3
MarkupSafe==1.1.1
ovirt-engine-sdk-python==4.3.0
psutil==5.6.5
pycparser==2.19
pycurl==7.43.0.3
python-memcached==1.59
PyYAML==5.1.2
six==1.13.0
(custom-venv)# python -V
Python 2.7.16

And from the UI Settings-> Systems I have adjusted the virtual environment to point to /opt/new

Now when I run any Job, I am getting the following error :
Traceback (most recent call last):
File "/opt/new/custom-venv/bin/ansible-playbook", line 60, in
import ansible.constants as C
File "/opt/new/custom-venv/lib/python2.7/site-packages/ansible/constants.py", line 12, in
from jinja2 import Template
File "/opt/new/custom-venv/lib/python2.7/site-packages/jinja2/init.py", line 33, in
from jinja2.environment import Environment, Template
File "/opt/new/custom-venv/lib/python2.7/site-packages/jinja2/environment.py", line 13, in
import weakref
File "/opt/rh/python27/root/usr/lib64/python2.7/weakref.py", line 14, in
from _weakref import (
ImportError: cannot import name _remove_dead_weakref

How can I fix it so I can run jobs using the virtual environment?