Madrona Venture Group has been a mainstay of the Seattle and northwest United States startup ecosystem for years now, and it looks like it is going to continue that legacy going forward. In a filing with the SEC, the firm announced its eighth venture capital fund, raising $320 million. That’s up slightly from the firm’s past two funds, both of which were $300 million vehicles. The filing comes roughly a year and a half after the firm’s last fundraise for its seventh fund. That slight increase in size is a…