Matplotlib-pgfutils

This release adds support for Python 3.10.

It also clarifies the supported versions of Python (following NEP29, any minor version released within the 42 months prior to a pgfutils release) and Matplotlib (any minor version released within the 24 months prior to a pgfutils release). This is currently Python 3.7 to 3.10 and Matplotlib 3.1 to 3.5. The latest point release of these minor versions are used for continuous integration testing.

As a result of this, an obsolete workaround for setting the preamble in older versions of Matplotlib has been removed as it was incompatible with the latest release.

This adds support for setting environment variables through the pgfutils.environment configuration option.

This release adds some new features:

* Add configuration option to customise the Python path to make
local libraries available to scripts.

* Allow configuration to specify which directories might contain data
files (for the file tracking feature).

* File tracking can now also track imported Python files in configured
directories.

Fix a bug in one of the tests which failed if netCDF4 was not installed.

Support for tracking netCDF4 dependencies, and adding other
dependencies manually. Some documentation improvements are also
included. All examples and included code now specifically call python3
rather than python.

Some fixes for the next Matplotlib release and some fixes to the documentation.