Removed Python 2 and Python 3.5- support

Deprecation of Python versions below 3.6 gives opportunity to unify the
build and install requirements for OpenZFS packages. The minimal
supported Python version is 3.6 as this is the most recent Python
package CentOS/RHEL 7 users can get.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12925

cmd/arc_summary/Makefile.am

@@ -1,13 +1,8 @@
 bin_SCRIPTS = arc_summary
 
 CLEANFILES = arc_summary
-EXTRA_DIST = arc_summary2 arc_summary3
-
-if USING_PYTHON_2
-SCRIPT = arc_summary2
-else
+EXTRA_DIST = arc_summary3
 SCRIPT = arc_summary3
-endif
 
 arc_summary: $(SCRIPT)
 	cp $< $@

cmd/arc_summary/arc_summary3

@@ -191,21 +191,13 @@ elif sys.platform.startswith('linux'):
         # there, so we fall back on modinfo
         command = ["/sbin/modinfo", request, "-0"]
 
-        # The recommended way to do this is with subprocess.run(). However,
-        # some installed versions of Python are < 3.5, so we offer them
-        # the option of doing it the old way (for now)
         info = ''
 
         try:
 
-            if 'run' in dir(subprocess):
-                info = subprocess.run(command, stdout=subprocess.PIPE,
-                                      universal_newlines=True)
-                raw_output = info.stdout.split('\0')
-            else:
-                info = subprocess.check_output(command,
-                                               universal_newlines=True)
-                raw_output = info.split('\0')
+            info = subprocess.run(command, stdout=subprocess.PIPE,
+                                  check=True, universal_newlines=True)
+            raw_output = info.stdout.split('\0')
 
         except subprocess.CalledProcessError:
             print("Error: Descriptions not available",