Python3: replace distutils with sysconfig

- `distutils` module is long time deprecated and already deleted
  from the CPython mainline.

- To remain compatible with Debian/Ubuntu Python3 packaging style,
  try
  `distutils.sysconfig.get_python_path(0,0)`
  first with fallback on
  `sysconfig.get_path('purelib')`

- pyzfs_unittest suite is run unconditionally as a part of ZTS.

- Add pyzfs_unittest suite to sanity tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12833 
Closes #13280 
Closes #14177

tests/zfs-tests/tests/functional/pyzfs/pyzfs_unittest.ksh.in

@@ -18,6 +18,8 @@
 
 if [ -n "$ASAN_OPTIONS" ]; then
 	export LD_PRELOAD=$(ldd "$(command -v zfs)" | awk '/libasan\.so/ {print $3}')
+	# ASan reports leaks in CPython 3.10
+	ASAN_OPTIONS="$ASAN_OPTIONS:detect_leaks=false"
 fi
 
 #
@@ -30,20 +32,6 @@ fi
 #
 
 verify_runnable "global"
-
-# Verify that the required dependencies for testing are installed.
-@PYTHON@ -c "import cffi" 2>/dev/null ||
-	log_unsupported "python3-cffi not found by Python"
-
-# We don't just try to "import libzfs_core" because we want to skip these tests
-# only if pyzfs was not installed due to missing, build-time, dependencies; if
-# we cannot load "libzfs_core" due to other reasons, for instance an API/ABI
-# mismatch, we want to report it.
-@PYTHON@ -c '
-import pkgutil, sys
-sys.exit(pkgutil.find_loader("libzfs_core") is None)' ||
-	log_unsupported "libzfs_core not found by Python"
-
 log_assert "Verify the nvlist and libzfs_core Python unittest run successfully"
 
 # log_must buffers stderr, which interacts badly with