Canonicalize Python shebangs

/usr/bin/env python3 is the suggested[1] shebang for Python in general
(likewise for python2) and is conventional across platforms. This eases
development on systems where python is not installed in /usr/bin
(FreeBSD for example) and makes it possible to develop in virtual
environments (venv) for isolating dependencies.

Many packaging guidelines discourage the use of /usr/bin/env, but since
this is the canonical way of writing shebangs in the Python community,
many packaging scripts are already equipped to handle substituting the
appropriate absolute path to python automatically.

Some RPM package builders lacking brp-mangle-shebangs need a small
fallback mechanism in the package spec to stamp the appropriate shebang
on installed Python scripts.

[1]: https://docs.python.org/3/using/unix.html?#miscellaneous

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9314

rpm/generic/zfs.spec.in

@@ -53,10 +53,6 @@
 %bcond_with    asan
 %bcond_with    systemd
 
-# Exclude test-runner.py from the rpmbuild shebang check to allow it to run
-# under Python 2 and 3.
-%global __brp_mangle_shebangs_exclude_from test-runner.py
-
 # Generic enable switch for systemd
 %if %{with systemd}
 %define _systemd 1
@@ -354,6 +350,14 @@ make %{?_smp_mflags}
 %{__rm} -rf $RPM_BUILD_ROOT
 make install DESTDIR=%{?buildroot}
 find %{?buildroot}%{_libdir} -name '*.la' -exec rm -f {} \;
+%if 0%{!?__brp_mangle_shebangs:1}
+find %{?buildroot}%{_bindir} \
+    \( -name arc_summary -or -name arcstat -or -name dbufstat \) \
+    -exec %{__sed} -i 's|^#!.*|#!%{__python}|' {} \;
+find %{?buildroot}%{_datadir} \
+    \( -name test-runner.py -or -name zts-report.py \) \
+    -exec %{__sed} -i 's|^#!.*|#!%{__python}|' {} \;
+%endif
 
 %post
 %if 0%{?_systemd}