pyln: Make the pytest requirement less stringent

contrib/pyln-testing/requirements.txt

@@ -1,4 +1,4 @@
-pytest==5.3.1
+pytest>=5.3.1
 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>=5.3.1
 python-bitcoinlib==0.10.2
 recommonmark==0.5.0
 sphinx-rtd-theme==0.4.2