utilities/runtests: Fix doctest runner for Python 3
utilities/runtests: Fix doctest runner for Python 3

In Python 3, the _load_testfile method has a fourth argument, encoding.
As we already use this, just pass it to the function when under Python
3. However, we then try to decode a str (because we specified
an encoding) which is wrong, so set the encoding to None in Python 3. We
cannot just use None as an encoding as Python 2 relies on the decode
step.

This error only appeared when running .txt doctests, as the usual
doctests use a different codepath.

yashsharan/sympy,aktech/sympy,Vishluck/sympy,wyom/sympy,saurabhjn76/sympy,meghana1995/sympy,mcdaniel67/sympy,AunShiLord/sympy,srjoglekar246/sympy,emon10005/sympy,emon10005/sympy,MridulS/sympy,maniteja123/sympy,jaimahajan1997/sympy,Gadal/sympy,hrashk/sympy,ahhda/sympy,kevalds51/sympy,madan96/sympy,Arafatk/sympy,hrashk/sympy,emon10005/sympy,moble/sympy,aktech/sympy,ChristinaZografou/sympy,maniteja123/sympy,lidavidm/sympy,beni55/sympy,Designist/sympy,shipci/sympy,kaushik94/sympy,atsao72/sympy,Shaswat27/sympy,jaimahajan1997/sympy,madan96/sympy,kmacinnis/sympy,amitjamadagni/sympy,vipulroxx/sympy,mcdaniel67/sympy,yukoba/sympy,drufat/sympy,Vishluck/sympy,grevutiu-gabriel/sympy,shikil/sympy,madan96/sympy,cccfran/sympy,abloomston/sympy,lindsayad/sympy,grevutiu-gabriel/sympy,MridulS/sympy,Gadal/sympy,ga7g08/sympy,liangjiaxing/sympy,kaushik94/sympy,mafiya69/sympy,souravsingh/sympy,Arafatk/sympy,atreyv/sympy,rahuldan/sympy,bukzor/sympy,farhaanbukhsh/sympy,abhiii5459/sympy,abhiii5459/sympy,abhiii5459/sympy,wanglongqi/sympy,rahuldan/sympy,dqnykamp/sympy,jamesblunt/sympy,sunny94/temp,moble/sympy,Shaswat27/sympy,meghana1995/sympy,kevalds51/sympy,atsao72/sympy,saurabhjn76/sympy,asm666/sympy,Titan-C/sympy,ChristinaZografou/sympy,VaibhavAgarwalVA/sympy,wyom/sympy,Designist/sympy,cswiercz/sympy,chaffra/sympy,VaibhavAgarwalVA/sympy,iamutkarshtiwari/sympy,MechCoder/sympy,mafiya69/sympy,ChristinaZografou/sympy,Davidjohnwilson/sympy,kaichogami/sympy,mcdaniel67/sympy,vipulroxx/sympy,debugger22/sympy,cccfran/sympy,Designist/sympy,sahilshekhawat/sympy,yukoba/sympy,liangjiaxing/sympy,oliverlee/sympy,sahilshekhawat/sympy,sahmed95/sympy,lidavidm/sympy,atsao72/sympy,yashsharan/sympy,Sumith1896/sympy,sunny94/temp,Curious72/sympy,AunShiLord/sympy,shikil/sympy,farhaanbukhsh/sympy,souravsingh/sympy,atreyv/sympy,kumarkrishna/sympy,ahhda/sympy,kmacinnis/sympy,Davidjohnwilson/sympy,Davidjohnwilson/sympy,jamesblunt/sympy,debugger22/sympy,Titan-C/sympy,maniteja123/sympy,kumarkrishna/sympy,aktech/sympy,ga7g08/sympy,jaimahajan1997/sympy,lindsayad/sympy,skidzo/sympy,shipci/sympy,chaffra/sympy,MechCoder/sympy,Arafatk/sympy,Sumith1896/sympy,diofant/diofant,lindsayad/sympy,AkademieOlympia/sympy,hrashk/sympy,garvitr/sympy,Mitchkoens/sympy,AkademieOlympia/sympy,asm666/sympy,Curious72/sympy,skidzo/sympy,bukzor/sympy,VaibhavAgarwalVA/sympy,jbbskinny/sympy,cswiercz/sympy,sampadsaha5/sympy,MridulS/sympy,sampadsaha5/sympy,kaichogami/sympy,jbbskinny/sympy,meghana1995/sympy,pbrady/sympy,hargup/sympy,jerli/sympy,beni55/sympy,souravsingh/sympy,drufat/sympy,garvitr/sympy,vipulroxx/sympy,oliverlee/sympy,abloomston/sympy,liangjiaxing/sympy,toolforger/sympy,iamutkarshtiwari/sympy,kumarkrishna/sympy,jamesblunt/sympy,kaushik94/sympy,postvakje/sympy,shipci/sympy,saurabhjn76/sympy,rahuldan/sympy,pbrady/sympy,cccfran/sympy,moble/sympy,toolforger/sympy,chaffra/sympy,dqnykamp/sympy,farhaanbukhsh/sympy,Gadal/sympy,Titan-C/sympy,wanglongqi/sympy,cswiercz/sympy,flacjacket/sympy,skidzo/sympy,kaichogami/sympy,jerli/sympy,pandeyadarsh/sympy,AunShiLord/sympy,mafiya69/sympy,jerli/sympy,ahhda/sympy,sahilshekhawat/sympy,Mitchkoens/sympy,jbbskinny/sympy,dqnykamp/sympy,postvakje/sympy,ga7g08/sympy,pbrady/sympy,beni55/sympy,AkademieOlympia/sympy,debugger22/sympy,sampadsaha5/sympy,drufat/sympy,lidavidm/sympy,toolforger/sympy,skirpichev/omg,atreyv/sympy,sahmed95/sympy,garvitr/sympy,postvakje/sympy,Sumith1896/sympy,sahmed95/sympy,pandeyadarsh/sympy,oliverlee/sympy,sunny94/temp,kevalds51/sympy,hargup/sympy,amitjamadagni/sympy,bukzor/sympy,Mitchkoens/sympy,Shaswat27/sympy,asm666/sympy,wanglongqi/sympy,Vishluck/sympy,MechCoder/sympy,iamutkarshtiwari/sympy,kmacinnis/sympy,yashsharan/sympy,abloomston/sympy,wyom/sympy,grevutiu-gabriel/sympy,yukoba/sympy,pandeyadarsh/sympy,Curious72/sympy,hargup/sympy,shikil/sympy