Docutils: Documentation Utilities Code

Removed item about backslash-escaped colons in field list field names: implemented some time ago.

* Consolidated version identifier documentation, moving text from docutils/__init__.py to docs/dev/policies.txt.
* Cleaned up version-related code in docutils/__init__.py.
* Reworked the Version Numbering section in docs/dev/policies.txt.
* Added test/test_utils.HelperFunctionTests.test_implicit_version_identifier to test the implicit use of docutils.utils.version_identifier().

Test __version_info__.releaselevel values.

Also rename the test functions according to the scheme and replace
a comment in the docstring with explicit code.

* Consolidated version numbering documentation in docs/dev/policies.txt (moved text from docs/dev/release.txt).
* Updated docutils.utils.version_identifier for Python 2.6 (docutils.VersionInfo).
* Moved code from tests/test/test_utils.py::HelperFunctionsTests.test_version_identifier to tests/test__init__.py::VersionInfoTests.test_version_identifier; uncommented lines in tests/test__init__.py::VersionInfoTests.test_version_info.
* Added new tests/test/test_utils.py::HelperFunctionsTests.test_version_identifier.

Drop 2.4 and 2.5 compatibility code, part 2.

Drop compatibility code for Python 2.4 and 2.5.

Documentation update: Dropped support for Python 2.4, 2.5, 3.1, and 3.2.

No change to the code yet, but from now on constructs requiring 2.6 or later
can be checked in.

Fix [ 323 ]: do not add ``\\\\phantomsection`` and whitespace to ``parts["title"].

Update documentation of the version identification.

Document the consensus reached about version identifier and version info tuple
in the discussion on docutils-devel in Mai and June 2017.

change version to 0.15b.dev