Use the optional Pythran build dependency

scipy.spec

@@ -1,6 +1,11 @@
 # without means enabled
 %bcond_with doc
 
+# Pythran is an optional build dependency.
+# When used, it makes some modules faster,
+# but it is usually not available soon enough for new major Python versions.
+%bcond_with pythran
+
 # Set to pre-release version suffix if building pre-release, else %%{nil}
 %global rcver %{nil}
 
@@ -15,7 +20,7 @@
 Summary:    Scientific Tools for Python
 Name:       scipy
 Version:    1.7.0
-Release:    1%{?dist}
+Release:    2%{?dist}
 
 # BSD -- whole package except:
 # Boost -- scipy/special/cephes/scipy_iv.c
@@ -39,6 +44,10 @@ BuildRequires:  python3-pytest
 BuildRequires:  python3-pytest-xdist
 BuildRequires:  python3-pytest-timeout
 
+%if %{with pythran}
+BuildRequires:  pythran
+%endif
+
 %if %{with doc}
 BuildRequires:  python3-sphinx
 BuildRequires:  python3-matplotlib
@@ -101,7 +110,7 @@ rm doc/sphinxext -r
 rm $(grep -rl '/\* Generated by Cython') PKG-INFO
 
 %build
-export SCIPY_USE_PYTHRAN=0
+export SCIPY_USE_PYTHRAN=0%{?with_pythran}
 
 for PY in %{python3_version}; do
   # Adding -fallow-argument-mismatch workaround for https://github.com/scipy/scipy/issues/11611
@@ -127,7 +136,7 @@ for PY in %{python3_version}; do
 done
 
 %install
-export SCIPY_USE_PYTHRAN=0
+export SCIPY_USE_PYTHRAN=0%{?with_pythran}
 
 %py3_install
 # Some files got ambiguous python shebangs, we fix them after everything else is done
@@ -189,6 +198,9 @@ popd
 %endif
 
 %changelog
+* Wed Jul 14 2021 Miro Hrončok <mhroncok@redhat.com> - 1.7.0-2
+- Use the optional Pythran build dependency
+
 * Wed Jun 23 2021 Nikola Forró <nforro@redhat.com> - 1.7.0-1
 - New upstream release 1.7.0
   resolves: #1953422