Release version 1.6.0.

* Python 3.10 compatibility.

* Clarify supported Python and Matplotlib versions.

* Drop obsolete workaround for setting the preamble.

Author: bcbnz

pgfutils.py

@@ -36,7 +36,7 @@
 
 """
 
-__version__ = "1.6.0-alpha0"
+__version__ = "1.6.0"
 
 # We don't import Matplotlib here as this brings in NumPy. In turn, NumPy
 # caches a reference to the io.open() method as part of its data loading