Fix pytest major version

They released version 6, which is causing some trouble apparently. So fix it
to v5.

contrib/pyln-testing/requirements.txt

@@ -1,4 +1,4 @@
-pytest>=5.3.1
+pytest==5.*
 Flask==1.1.1
 cheroot==8.2.1
 ephemeral-port-reserve==1.1.1

requirements.txt

@@ -17,7 +17,7 @@ pytest-benchmark==3.2.2
 pytest-forked==1.0.2
 pytest-timeout==1.3.3
 pytest-xdist==1.29.0
-pytest>=5.3.1
+pytest == 5.*
 python-bitcoinlib==0.10.2
 recommonmark>=0.6.0
 sphinx-rtd-theme==0.4.2