Fix strange fc26-only python2 packaging issue

For some reaon, /usr/bin/python, which should be symlink to python2,
is not available on Fedora 26 COPR build servers (it seems to be on
"normal" installations).

From COPR targets, it really only affects Fedora 26 in particular: 25
was OK, and 27 is OK again, just as are EPELs and RHELs.

packaging/template.spec

 BuildRequires: %shellfu_req
 BuildRequires: libxml2-python
 BuildRequires: shellfu-bash-pretty
+%if 0%{?fedora} == 26
+BuildRequires: /usr/bin/python
+%endif
 
 Requires: %shellfu_req
 Requires: libxml2-python
 Requires: shellfu-bash
 Requires: shellfu-bash-pretty
+%if 0%{?fedora} == 26
+BuildRequires: /usr/bin/python
+%endif
 %description
 pxpath is Shellfu/Bash wrapper around libxml2-python with intent
 to enable you to make simple XPath queries from a shell script,