Fix Python 3.5 compat: capture_output and text= not available

Use stdout=PIPE/stderr=PIPE and manual .decode() instead.
Use official 1.6.6 gplan.so (built with Stretch-era GCC, no GLIBC_2.29 dep).

src/py/bbctrl/Hooks.py

@@ -359,13 +359,15 @@ class Hooks:
         result = subprocess.run(
             command, shell=True, env=env,
             timeout=timeout,
-            capture_output=True, text=True
+            stdout=subprocess.PIPE, stderr=subprocess.PIPE
         )
 
-        if result.stdout.strip():
-            self.log.info('Script stdout: %s' % result.stdout.strip())
+        stdout = result.stdout.decode('utf-8', errors='replace').strip()
+        stderr = result.stderr.decode('utf-8', errors='replace').strip()
+
+        if stdout:
+            self.log.info('Script stdout: %s' % stdout)
 
         if result.returncode != 0:
             raise Exception('Script failed (%d): %s' %
-                            (result.returncode,
-                             result.stderr.strip() or 'non-zero exit'))
+                            (result.returncode, stderr or 'non-zero exit'))