Fix stdapi_sys_config_getenv for Python3

Author: zeroSteiner

data/meterpreter/ext_server_stdapi.py

@@ -675,8 +675,10 @@ def channel_open_stdapi_net_tcp_server(request, response):
 @meterpreter.register_function
 def stdapi_sys_config_getenv(request, response):
 	for env_var in packet_enum_tlvs(request, TLV_TYPE_ENV_VARIABLE):
-		pgroup = ''
-		env_var = env_var['value'].translate(None, '%$')
+		pgroup = bytes()
+		env_var = env_var['value']
+		env_var = env_var.replace('%', '')
+		env_var = env_var.replace('$', '')
 		env_val = os.environ.get(env_var)
 		if env_val:
 			pgroup += tlv_pack(TLV_TYPE_ENV_VARIABLE, env_var)