Pin build to Python 3.10

Also make it possible to override it using the PYTHON make variable:

    $ make PYTHON="$HOME/bin/python3.10"

Fixes hpyproject#45

Makefile

@@ -2,15 +2,16 @@ SHELL := bash
 TARGET ?= origin
 # hpy static page and blog makefile
 # type `make help` to see all options 
+PYTHON := python3.10
 
 all: build
 
 .PHONY: clean build help
 
 
 venv_nikola/bin/nikola:  ## create a virtualenv to build the website
-	@virtualenv -ppython3 ./venv_nikola
-	@venv_nikola/bin/python -mpip install nikola==8.2.2 markdown==3.2.2 jinja2 aiohttp watchdog ruamel.yaml feedparser
+	@virtualenv -p $(PYTHON) ./venv_nikola
+	@venv_nikola/bin/python -m pip install -r requirements.txt
 	@venv_nikola/bin/nikola plugin -i sidebar
 	@venv_nikola/bin/nikola plugin -i localsearch
 

requirements.txt

@@ -0,0 +1,7 @@
+aiohttp
+feedparser
+jinja2
+markdown==3.2.2
+nikola==8.2.2
+ruamel.yaml
+watchdog