Ethics of scraping "public" data sources to obtain email addresses
A relevant guideline from the Council of American Survey Research Organizations' Code of Standards and Ethics for Survey Research:


  Research Organizations are required to verify that individuals contacted for research by email have
  a reasonable expectation that they will receive email contact for research. Such agreement can be
  assumed when ALL of the following conditions exist:
  
  
  A substantive pre-existing relationship exists between the individuals contacted and the
  Research Organization, the Client supplying email addresses, or the Internet Sample Providers
  supplying the email addresses (the latter being so identified in the email invitation);
  Survey email invitees have a reasonable expectation, based on the pre-existing relationship
  where survey email invitees have specifically opted in for Internet research with the research
  company or Sample Provider, or in the case of Client-supplied lists that they may be contacted
  for research and invitees have not opted out of email communications;
  Survey email invitations clearly communicate the name of the sample provider, the relationship
  of the individual to that provider, and clearly offer the choice to be removed from future email
  contact.
  The email sample list excludes all individuals who have previously requested removal from
  future email contact in an appropriate and timely manner.
  Participants in the email sample were not recruited via unsolicited email invitations.
  


It would seem here that GitHub users do not have a reasonable expectation that they will be contacted for research.