Updates user agent to fix 403 on Ropsten

Ropsten seems to be blacklisting `python-requests` user agent, refs:
- AndreMiras/EtherollApp#156
- https://www.reddit.com/r/etherscan/comments/dtg8xl/
- corpetty/py-etherscan-api#70

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Change Log
 
+## [Unreleased]
+
+  - Fix Ropsten 403 errors
+
+
 ## [20191107]
 
   - API key refactoring

pyetheroll/etheroll.py

@@ -37,6 +37,9 @@
     # as we still want some very outdate data to get wiped at some point
     "expire_after": 30 * 24 * 60 * 60,
 }
+REQUESTS_HEADERS = {
+    "User-Agent": "https://github.com/AndreMiras/pyetheroll",
+}
 
 
 def abi_definitions(contract_abi, typ):
@@ -60,6 +63,15 @@ def merge_logs(bet_logs, bet_results_logs):
     return merged_logs
 
 
+def update_user_agent(headers):
+    """
+    Default `requests` user agent is blocked on Ropsten, refs:
+      - https://github.com/corpetty/py-etherscan-api/issues/70
+      - https://www.reddit.com/r/etherscan/comments/dtg8xl/
+    """
+    return dict(headers, **REQUESTS_HEADERS)
+
+
 class Etheroll:
 
     _etheroll = None
@@ -97,6 +109,8 @@ def __init__(
             self.etherscan_contract_api = ChainEtherscanContract(
                 address=self.contract_address, api_key=self.etherscan_api_key
             )
+            self.etherscan_contract_api.http.headers = update_user_agent(
+                self.etherscan_contract_api.http.headers)
             self.contract_abi = json.loads(
                 self.etherscan_contract_api.get_abi()
             )
@@ -268,6 +282,8 @@ def get_transaction_page(
         etherscan_account_api = self.ChainEtherscanAccount(
             address=address, api_key=self.etherscan_api_key
         )
+        etherscan_account_api.http.headers = update_user_agent(
+            etherscan_account_api.http.headers)
         sort = "desc"
         try:
             transactions = etherscan_account_api.get_transaction_page(
@@ -592,6 +608,8 @@ def get_balance(self, address):
         etherscan_account_api = self.ChainEtherscanAccount(
             address=address, api_key=self.etherscan_api_key
         )
+        etherscan_account_api.http.headers = update_user_agent(
+            etherscan_account_api.http.headers)
         balance_wei = int(etherscan_account_api.get_balance())
         balance_eth = round(balance_wei / 1e18, ROUND_DIGITS)
         return balance_eth

tests/test_etheroll.py

@@ -1128,6 +1128,7 @@ def test_get_transaction_page(self):
         contract_address = "0x048717Ea892F23Fb0126F00640e2b18072efd9D2"
         expected_transactions = mock.sentinel
         m_ChainEtherscanAccount = mock.Mock(spec=EtherscanAccount)
+        m_ChainEtherscanAccount.return_value.http.headers = {}
         m_ChainEtherscanAccount.return_value.get_transaction_page = mock.Mock(
             return_value=expected_transactions
         )