Drop support for Python 3.6 (#1004)

* Drop support for Python 3.6
* Remove unused time parsing function
* Use Python 3.7 datetime function for writing stats data conversion
* Test against Python 3.10 on macOS and Windows

novelwriter/core/toodt.py

@@ -330,7 +330,7 @@ class ToOdt(Tokenizer):
 
         # Meta Data
         xMeta = etree.SubElement(self._xMeta, _mkTag("meta", "creation-date"))
-        xMeta.text = datetime.now().strftime(r"%Y-%m-%dT%H:%M:%S")
+        xMeta.text = datetime.now().isoformat(sep="T", timespec="seconds")
 
         xMeta = etree.SubElement(self._xMeta, _mkTag("meta", "generator"))
         xMeta.text = f"novelWriter/{novelwriter.__version__}"