pyln-testing: require bitcoin v0.20 for PSBT handling, and create wallet for v0.21

With older bitcoind, PSBTs fail:

```
    def test_utxopsbt(node_factory, bitcoind, chainparams):
...
>       psbt = bitcoind.rpc.decodepsbt(funding['psbt'])

tests/test_wallet.py:561:
...
self = <bitcoin.rpc.RawProxy object at 0x7f4ec602e100>, service_name = 'decodepsbt'
args = ('cHNidP8BADMCAAAAAaoMihSVXlpdBHGcJePiroqtwq/b1zu09j8IkTG4OKs7AQAAAAD9////AGYAAAAAAQDeAgAAAAABAefqB6BkZE1/AqXaf36T02a7.../7Stf971PEgvUXgvASECXPTIO6tIVxDih6tfKy6suj6WJhhjycwoaTeuso/AQ8llAAAAAQEfQEIPAAAAAAAWABQB+tkKvNZml+JZIWRyLeSpXr7hZQA=',)
postdata = '{"version": "1.1", "method": "decodepsbt", "params": ["cHNidP8BADMCAAAAAaoMihSVXlpdBHGcJePiroqtwq/b1zu09j8IkTG4OKs7AQ...gvUXgvASECXPTIO6tIVxDih6tfKy6suj6WJhhjycwoaTeuso/AQ8llAAAAAQEfQEIPAAAAAAAWABQB+tkKvNZml+JZIWRyLeSpXr7hZQA="], "id": 1}'
headers = {'Authorization': b'Basic cnBjdXNlcjpycGNwYXNz', 'Content-type': 'application/json', 'Host': 'localhost', 'User-Agent': 'AuthServiceProxy/0.1'}
response = {'error': {'code': -22, 'message': 'TX decode failed PSBT is not sane.: iostream error'}, 'id': 1, 'result': None}
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: *** Requires bitcoind v0.20.1 or above ***

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -127,9 +127,15 @@ def bitcoind(directory, teardown_checks):
 
     info = bitcoind.rpc.getnetworkinfo()
 
-    if info['version'] < 160000:
+    # FIXME: include liquid-regtest in this check after elementsd has been
+    # updated
+    if info['version'] < 200100 and env('TEST_NETWORK') != 'liquid-regtest':
         bitcoind.rpc.stop()
-        raise ValueError("bitcoind is too old. At least version 16000 (v0.16.0)"
+        raise ValueError("bitcoind is too old. At least version 20100 (v0.20.1)"
+                         " is needed, current version is {}".format(info['version']))
+    elif info['version'] < 160000:
+        bitcoind.rpc.stop()
+        raise ValueError("elementsd is too old. At least version 160000 (v0.16.0)"
                          " is needed, current version is {}".format(info['version']))
 
     info = bitcoind.rpc.getblockchaininfo()