djtest: use pyelftools to find uwsgi ld.so

Change-Id: I54bdaa588ff15d8c6ca73c4307076a93a5682d78
diff --git a/third_party/py/requirements.txt b/third_party/py/requirements.txt
index 875712e..94765d7 100644
--- a/third_party/py/requirements.txt
+++ b/third_party/py/requirements.txt
@@ -33,6 +33,7 @@
 pyasn1==0.4.5
 pycparser==2.19
 PyNaCl==1.3.0
+pyelftools==0.26
 python-dateutil==2.8.0
 pytz==2019.1
 requests==2.22.0