fix: normalize pip requirements

This will change some `requirements.txt` of pyln-testing in a way that
it does not require different package version i.e. to `tests/requirements.txt`.

The reason for this is that users are not forced to hassle with pyenv
or virtualenv and could just use `--user`.

```bash
pip install --user -r tests/requirements.txt -r contrib/pyln-testing/requirements.txt
```

Changelog-None

contrib/pyln-testing/requirements.txt

@@ -1,6 +1,6 @@
-pytest==5.0.1
+pytest==5.3.1
 Flask==1.1.1
-cheroot==6.5.5
+cheroot==8.2.1
 ephemeral-port-reserve==1.1.1
-python-bitcoinlib==0.10.1
+python-bitcoinlib==0.10.2
 psycopg2-binary==2.8.4