atanas argirov

move best_match/2 closer to the source of data used for resolution decisions - PR pending.

On erldnsimple start: ```shell 2020-09-22 15:05:04 =ERROR REPORT==== Ranch listener erldns_admin, connection process , stream 1 had its request process exit with reason {badmap,[]} and stacktrace [{cowboy_req,reply,4,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_req.erl"},{line,785}]},{erldns_admin_zone_resource_handler,to_json,2,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/erldns_admin/src/erldns_admin_zone_resource_handler.erl"},{line,68}]},{cowboy_rest,call,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,1576}]},{cowboy_rest,set_resp_body,2,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,1466}]},{cowboy_rest,upgrade,4,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,288}]},{cowboy_stream_h,execute,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,296}]},{cowboy_stream_h,request_process,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,274}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,249}]}] 2020-09-22 15:11:32 =SUPERVISOR...

* refactored Dockerfile to account for asyncio's dependencies * using requirements.txt as for pip in container build * switched from pypy 3.7 to python 3.9.2 as base * amended README.md...

Refactoring calls to remove deprecated Crypto API calls.