Expose the environment variables used by %pytest via %{py3_test_envvars}

This way, we will be able to reuse them in %tox from pyproject-rpm-macros.
Packagers will be able to use them in their spec files.

See https://lists.fedoraproject.org/archives/list/python-devel@lists.fedoraproject.org/thread/A3QQSHP5OSLIVMCL52AR2GRBRXYQHU6B/

macros.python

@@ -131,6 +131,16 @@
   end
 }
 
+# Environment variables for testing used standalone, e.g.:
+#  %%{py_test_envvars} %%{python} -m unittest
+%py_test_envvars %{expand:\\\
+  CFLAGS="${CFLAGS:-${RPM_OPT_FLAGS}}" LDFLAGS="${LDFLAGS:-${RPM_LD_FLAGS}}"\\\
+  PATH="%{buildroot}%{_bindir}:$PATH"\\\
+  PYTHONPATH="${PYTHONPATH:-%{buildroot}%{python_sitearch}:%{buildroot}%{python_sitelib}}"\\\
+  PYTHONDONTWRITEBYTECODE=1\\\
+  %{?__pytest_addopts:PYTEST_ADDOPTS="${PYTEST_ADDOPTS:-} %{__pytest_addopts}"}\\\
+  PYTEST_XDIST_AUTO_NUM_WORKERS=%{_smp_build_ncpus}}
+
 %python_disable_dependency_generator() \
 %undefine __pythondist_requires \
 %{nil}