Add mempool.space option for mainnet block explorer (#6261)

* Add mempool.space

* Adds mempool.space testnet explorer

Didn't realize they had testnet as well - added

electrum/util.py

@@ -737,6 +737,8 @@ mainnet_block_explorers = {
                         {'tx': 'transaction/', 'addr': 'address/'}),
     'blockonomics.co': ('https://www.blockonomics.co/',
                         {'tx': 'api/tx?txid=', 'addr': '#/search?q='}),
+    'mempool.space': ('https://mempool.space/',
+                        {'tx': 'tx/', 'addr': 'address/'}),
     'OXT.me': ('https://oxt.me/',
                         {'tx': 'transaction/', 'addr': 'address/'}),
     'smartbit.com.au': ('https://www.smartbit.com.au/',
@@ -756,6 +758,8 @@ testnet_block_explorers = {
                        {'tx': 'tx/', 'addr': 'address/'}),
     'Blockstream.info': ('https://blockstream.info/testnet/',
                         {'tx': 'tx/', 'addr': 'address/'}),
+    'mempool.space': ('https://mempool.space/testnet/',
+                        {'tx': 'tx/', 'addr': 'address/'}),    
     'smartbit.com.au': ('https://testnet.smartbit.com.au/',
                        {'tx': 'tx/', 'addr': 'address/'}),
     'system default': ('blockchain://000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943/',