Docutils: Documentation Utilities Code

Start reconciling "colwidth" attribute definition and use.

First, backwards compatible changes get the handling of the
`<colspec>` element's "colwidth" attribute in line with the
definition in the Exchange Table Model.

The "colwidth" attribute is defined in the "Exchange Table Model"
as either a *proportional measure* or a *fixed measure*.
Currently, Docutils supports only unitless values and interpets them
as proportional measures while the Exchange Table Model interprets
unitless values as fixed measures with default unit “pt”!

The new optional argument "unit_pattern" for `nodes.parse_measure()`
allows configuring what is recognized as a unit.

The new validation function `nodes.validate_proportional()`
for the "colwidth" attribute also accepts conformant values like
"5.2*" (for 5.2 times the "unit proportion").
For backwards compatibility reasons, the attribute is still stored
as numerical value in Python doctree element instances.

New auxiliary method `nodes.colspec.propwith()`:
returns the "colwidth" attribute as number if it is a
proportional measure.

Announce future changes in the RELEASE_NOTES

Add/modify test cases.

Review conditional test cases.

Skip tests instead of deleting samples from `totests` dictionary if
requirements for the tested feature are missing.
This way we have a record in the test output.

Reformat a skip decorator and "assertRaises" block in test_get_parser_class.py.

Skip functional test that requires PIL if PIL is absent.

For Docutils, PIL is a recommendation, no requirement.

It is, however, required for the extraction of the image size
from an image file and a warning is written if it it missing.
This makes the output differ from the "expected" sample on
a functional test where the "scale" option of the "image"
directive is used.

The "length_units_html5" functional test is intended to provide
an example of the length unit support for visual inspection with
a HTML browser. There are additional unit tests for the
"scale" option with and without PIL, so we can safely skip
the functional test if PIL is missing on a system.

update user/manpage

set report level to 1 to check image INFO

Fix formatting

Store attribute values with datatype "yesorno" as `bool` values.

Use `bool` instead of `int` to store values of "yesorno" attributes in
Python doctree element instances.
This is the "natural" choice for "yesorno" and restores consistency
with the validating function `nodes.validate_yesorno()`.

In XML and pseudoxml output, the value is serialized as "0" or "1"
for backwards compatibility.

manpage image handling redo

remove warnings suppression from std tests

test manpage warning on image content