Fix breakage documented in bpo-32149.
This reverts commit d8d6b9122134f040cd5a4f15f40f6c9e3386db4d.

Rather than requiring the path to blurb and/or sphinx-build to be specified to the make rule, enhance the Doc/Makefile to look for each first in a virtual environment created by make venv and, if not found, look on the normal process PATH. This allows the Doc/Makefile to take advantage of an installed spinx-build or blurb and, thus, do the right thing most of the time. Also, make the directory for the venv be configurable and document the `make venv` target.

* Doc: Indicate the language

* Renaming version_switcher to switchers (to add language_switcher).

* Adding language switch.

* Doc switchers: Enhance readability of regex parsing versions.

* Doc switchers: Desambiguate the need of a replace(/\/+$/g, '') by proper naming.

* Doc switchers: py3k can't reach js, it's redirected server-side by nginx.

* Doc switchers: Examples matching actual regexes.

* Doc switchers: Better fallback on unexisting translated version.

* bpo-30052: Always regenerate cross-references

The patch for bpo-30052 changed the preferred link target
for :func:`bytes` and :func`bytearray` references to be the
respective type definitions rather than the corresponding
builtin function entries.

This patch changes the daily documentation builds to disable
the output caching in Sphinx, in order to ensure that
cross-reference changes like this one are reliably picked
up and applied automatically after merging.

* Reformat Doc/README.txt to Doc/README.rst

* Update mention of Doc/README.rst

* Update mention of README.txt to README.rst

* Make line fold

* rstlint ignore Doc/README.rst

* conf.py ignore Doc/README.rst

* Update issue tracker url in Docs/README.rst

rstlint.py run by "make check" doesn't support Python 2.

"make venv" runs "$(PYTHON) -m venv", whereas Python 2 doens't
provide the venv module: it's a module of Python 3 standard library.

* Fix some deprecation warnings in Doc/conf.py
* Fix an rst error in Misc/NEWS