python: fill default macroses per new packaging guidelines

[ignatenkobrain]

like %py3_build for build section

[ignatenkobrain]

the same for %install: %py3_install

[onionka]

This may be a problem, because we expand those macros to use it to build and install project for further analysis. See what you get, when these are expanded:

$ rpm --eval %py3_install

CFLAGS="-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic" /usr/bin/python3 setup.py  install -O1 --skip-build --root ~/rpmbuild/BUILDROOT/%{name}-%{version}-%{release}.x86_64 

$ rpm --eval %py3_build

CFLAGS="-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic" /usr/bin/python3 setup.py  build --executable="/usr/bin/python3 -s"