Add py.test-3 to the list of tested pytest names

Some operating systems (e.g. CentOS, OpenBSD) install the main pytest
script as "py.test-3".  Add that name to the list of names passed to
AC_PATH_PROGS() in order for pytest to be properly detected on a broader
range of operating systems.
77 jobs for !3555 with michal/do-not-install-python-packages-in-gitlab-ci-jobs
detached
Status Job ID Name Coverage
  Autoconf
canceled #898023
amd64 linux
autoreconf

 
  Precheck
canceled #898025
amd64 linux
clang-format

canceled #898026
amd64 linux
coccinelle

canceled #898024
amd64 linux
misc

 
  Build
canceled #898042
amd64 linux
clang:asan

canceled #898046
amd64 linux
clang:buster:amd64

canceled #898048
amd64 libvirt
clang:freebsd11.3:amd64

canceled #898049
amd64 libvirt
clang:freebsd12.1:amd64

canceled #898050
amd64 libvirt
clang:openbsd6.6:amd64

canceled #898044
amd64 linux
clang:tsan

canceled #898027
amd64 linux
gcc:alpine3.11:amd64

canceled #898041
amd64 linux
gcc:asan

canceled #898040
amd64 linux
gcc:bionic:amd64

canceled #898032
amd64 linux
gcc:buster:amd64

canceled #898028
amd64 linux
gcc:centos6:amd64

canceled #898029
amd64 linux
gcc:centos7:amd64

canceled #898030
amd64 linux
gcc:centos8:amd64

canceled #898038
amd64 linux
gcc:fedora31:amd64

canceled #898045
amd64 linux
gcc:mutexatomics

canceled #898034
amd64 linux
gcc:out-of-tree

canceled #898047
amd64 linux
gcc:pkcs11

canceled #898033
amd64 linux
gcc:sid:amd64

canceled #898035
arm64 linux
gcc:sid:arm64

canceled #898036
i386 linux
gcc:sid:i386

canceled #898031
amd64 linux
gcc:stretch:amd64

canceled #898043
amd64 linux
gcc:tsan

canceled #898037
amd64 linux
gcc:tumbleweed:amd64

canceled #898039
amd64 linux
gcc:xenial:amd64

 
  Unit
canceled #898065
amd64 linux
unit:clang:asan

canceled #898069
amd64 linux
unit:clang:buster:amd64

canceled #898071
amd64 libvirt
unit:clang:freebsd11.3:amd64

canceled #898072
amd64 libvirt
unit:clang:freebsd12.1:amd64

canceled #898067
amd64 linux allowed to fail
unit:clang:tsan

canceled #898051
amd64 linux
unit:gcc:alpine3.11:amd64

canceled #898064
amd64 linux
unit:gcc:asan

canceled #898063
amd64 linux
unit:gcc:bionic:amd64

canceled #898056
amd64 linux
unit:gcc:buster:amd64

canceled #898052
amd64 linux
unit:gcc:centos6:amd64

canceled #898053
amd64 linux
unit:gcc:centos7:amd64

canceled #898054
amd64 linux
unit:gcc:centos8:amd64

canceled #898061
amd64 linux
unit:gcc:fedora31:amd64

canceled #898068
amd64 linux
unit:gcc:mutexatomics

canceled #898070
amd64 linux
unit:gcc:pkcs11

canceled #898057
amd64 linux
unit:gcc:sid:amd64

canceled #898058
arm64 linux
unit:gcc:sid:arm64

canceled #898059
i386 linux
unit:gcc:sid:i386

canceled #898055
amd64 linux
unit:gcc:stretch:amd64

canceled #898066
amd64 linux allowed to fail
unit:gcc:tsan

canceled #898060
amd64 linux
unit:gcc:tumbleweed:amd64

canceled #898062
amd64 linux
unit:gcc:xenial:amd64

 
  System
canceled #898087
amd64 linux
system:clang:asan

canceled #898091
amd64 linux
system:clang:buster:amd64

canceled #898093
amd64 libvirt
system:clang:freebsd11.3:amd64

canceled #898094
amd64 libvirt
system:clang:freebsd12.1:amd64

canceled #898089
amd64 linux allowed to fail
system:clang:tsan

canceled #898073
amd64 linux
system:gcc:alpine3.11:amd64

canceled #898086
amd64 linux
system:gcc:asan

canceled #898085
amd64 linux
system:gcc:bionic:amd64

canceled #898078
amd64 linux
system:gcc:buster:amd64

canceled #898074
amd64 linux
system:gcc:centos6:amd64

canceled #898075
amd64 linux
system:gcc:centos7:amd64

canceled #898076
amd64 linux
system:gcc:centos8:amd64

canceled #898083
amd64 linux
system:gcc:fedora31:amd64

canceled #898090
amd64 linux
system:gcc:mutexatomics

canceled #898092
amd64 linux
system:gcc:pkcs11

canceled #898079
amd64 linux
system:gcc:sid:amd64

canceled #898080
arm64 linux
system:gcc:sid:arm64

canceled #898081
i386 linux
system:gcc:sid:i386

canceled #898077
amd64 linux
system:gcc:stretch:amd64

canceled #898088
amd64 linux allowed to fail
system:gcc:tsan

canceled #898082
amd64 linux
system:gcc:tumbleweed:amd64

canceled #898084
amd64 linux
system:gcc:xenial:amd64

 
  Docs
canceled #898095
amd64 linux
docs

 
  Postcheck
canceled #898099
amd64 linux
cppcheck

canceled #898096
amd64 linux
flake8

canceled #898097
amd64 linux
pylint

canceled #898098
amd64 linux
scan-build