Support fallback to python2 on some distributions

Turns out that even distro as new as RHEL7 does not still support
python3 properly.  This commit enables fallback to python2 on older
distros.

RPM part could be solved by rpm macros; Debian part is left to require
Python 3.4 (as per current stable); we can fix that later if need be.

packaging/template.spec

 %global sfincb %{_datadir}/shellfu/include-bash
 %global sfmodn __SHELLFU_MODNAME__
 %global shellfu_req shellfu >= 0.10.4, shellfu < 0.11
+%if 0%{?rhel} <= 7
+%global py_eq     python
+%global pylibs_req python-libs
+%else
+%global py_eq     python3
+%global pylibs_req python3-libs
+%endif
 
 Name:       __MKIT_PROJ_PKGNAME__
 Version:    __MKIT_PROJ_VERSION__
 BuildArch:  noarch
 BuildRequires: %shellfu_req
 BuildRequires: perl
-BuildRequires: python3
-BuildRequires: python3-libs
+BuildRequires: %py_eq
+BuildRequires: %pylibs_req
 BuildRequires: shellfu-bash-pretty
 
 Requires: %shellfu_req
 Requires: perl
-Requires: python3
-Requires: python3-libs
+Requires: %py_eq
+Requires: %pylibs_req
 Requires: shellfu-bash
 Requires: shellfu-bash-pretty
 %description

src/uripecker.sh.skel

         echo 'for uri in re.findall(os.environ["URI_RE"], sys.stdin.read()):'
         echo '    print(uri)'
     )
-    URI_RE=$uri_re python3 -c "$py_code"
+    URI_RE=$uri_re $__URIPECKER_PYBIN -c "$py_code"
 }
 
 __uripecker__minimize() {
     local query=$1
     debug -v query
     #shellcheck disable=SC2028
-    LC_ALL=en_US.UTF-8 python3 -c "$(
-        echo 'import urllib.parse'
+    LC_ALL=en_US.UTF-8 $__URIPECKER_PYBIN -c "$(
+        echo "import $__URIPECKER_PYMOD"
         echo 'import sys'
-        echo 'print(urllib.parse.quote_plus(sys.argv[1]))'
+        echo "print($__URIPECKER_PYMOD.quote_plus(sys.argv[1]))"
     )" "$query"
     #FIXME: There should be proper way w/o touching LC_ALL
 }
 
+__shellfu_uripecker__init() {
+    #
+    # See what python version is here
+    #
+    if python3 --version 2>/dev/null \
+        | grep -qF 'Python 3.'; then
+        __URIPECKER_PYBIN=python3
+        __URIPECKER_PYMOD=urllib.parse
+    else
+        __URIPECKER_PYBIN=python
+        __URIPECKER_PYMOD=urllib
+    fi
+}
+
 #shellfu module-version=__MKIT_PROJ_VERSION__