FreeBSD: Update python package references

Install the "python" package to get the a friendly bin/python wrapper
and install dependencies using origins rather than package names in
order to install the default flavor.

docs/Getting Started/FreeBSD.rst

@@ -56,8 +56,8 @@ The following dependencies are required to build OpenZFS on FreeBSD:
 -  Optional packages for build:
    ::
 
-      pkg install python38 # or your preferred Python version
-      pkg install py38-sysctl # needed for arcstat, arc_summary, dbufstat
+      pkg install python
+      pkg install devel/py-sysctl # needed for arcstat, arc_summary, dbufstat
 
 -  Packages for checks and tests:
    ::
@@ -70,8 +70,7 @@ The following dependencies are required to build OpenZFS on FreeBSD:
           hs-ShellCheck \
           ksh93 \
           pamtester \
-          py38-flake8 \
-          python38 \
+          devel/py-flake8 \
           sudo
 
    Your preferred python version may be substituted. The user for