Update instructions for publishing releases of the Python runtime

* Added instructions on how to install the tools needed to build and
  publish the Python runtime.
* Fixed formatting of __version__ variable name (the double underscores
  were previously recognized as Asciidoc italics syntax).
* Added step for deleting old built distributions before building new
  ones, so that files left over from previous builds can't be uploaded
  by accident.
* Added build of wheel in addition to source distribution to the build
  step (see kaitai-io/kaitai_struct_python_runtime#25).
* Added `twine check` step to perform basic checks on the distributions
  before uploading.
* Changed upload instructions from the deprecated `setup.py upload`
  command to the now recommended `twine upload`.
* Updated instructions for providing credentials when uploading. They
  now no longer need to be stored in an insecure plain text file.
* Updated PyPI URLs.
* Changed `git push` command so that only the just created tag is
  pushed, and not any other tags that might exist locally.

Author: dgelessus

developers.adoc

@@ -225,25 +225,37 @@ to `X.Y.Z` format (e.g. `0.9.0` for 0.9 release)
 
 === Python
 
-* Create a `~/.pypirc` file with this (https://github.com/pypa/setuptools/issues/941#issuecomment-279123202[source]):
+* Make sure that the needed Python packaging tools are installed and up-to-date:
+** Debian/Ubuntu/etc.: `sudo apt install python3-setuptools python3-wheel twine`
+** Other Linuxes: see https://packaging.python.org/guides/installing-using-linux-tools/[Python Packaging User Guide]
+** Everywhere else: use your system package manager, or `python3 -m pip install --upgrade setuptools twine wheel`
+* Pump version in `kaitaistruct.py`, seek `\\__version__ =`
+* Build distributions and upload them to PyPI:
 +
-[source,ini]
+[source,bash]
 ----
-[distutils]
-index-servers =
-    pypi
-
-[pypi]
-repository: https://pypi.python.org/pypi
-username: <login>
-password: <pass>
+# Delete any old previously built distributions.
+$ rm -r dist/
+# Build source and wheel distributions.
+$ python3 setup.py sdist bdist_wheel
+# Run basic correctness checks on the built distributions.
+$ twine check dist/*
+# Upload the built distributions.
+# You may want to upload to TestPyPI first as a test before uploading to the real PyPI.
+# `twine upload` will prompt for credentials,
+# or you can provide them via the environment variables TWINE_USERNAME and TWINE_PASSWORD
+# (see https://twine.readthedocs.io/en/latest/#environment-variables).
+$ twine upload --repository=testpypi dist/* # Upload to TestPyPI (https://test.pypi.org/)
+$ twine upload dist/* # Upload to live PyPI (https://pypi.org/)
+----
+* Check that new version appears under https://pypi.org/project/kaitaistruct/#history
+* Create and push a Git tag for the new version:
++
+[source,bash]
+----
+$ git tag $VERSION
+$ git push origin $VERSION
 ----
-* Pump version in `kaitaistruct.py`, seek `__version__ =`
-* `python3 setup.py sdist upload`
-** (use `python3 setup.py sdist upload -r pypitest` to publish to testing server)
-* Check that new version appears at https://pypi.python.org/pypi/kaitaistruct[https://pypi.python.org/pypi/kaitaistruct]`/$VERSION`
-* `git tag $VERSION`
-* `git push --tags`
 
 === Ruby