PEP 458: update references to python-tuf repo (#2240)

In preparation for the 1.0.0** release of the TUF reference
implementation `python-tuf` documentation referenced in this PEP is
being moved. This patch updates the corresponding links in the PEP.

** See 059bfda391/docs/1.0.0-ANNOUNCEMENT.md

More specifically, the following link changes are performed:

- METADATA.md
  to: metadata format section in TUF spec
  Note: alternatively this could link to the new location of
  METADATA.md (see theupdateframework/python-tuf#1769), but the spec
  seems like a better resource

- TUTORIAL.md#repo-management
  to: new metadata API doc on readthedocs and new repo
  example/tutorial

- TUTORIAL.md#lazy-bin-walk
  to: new hashed bin delegation example/tutorial

- client implementation
  to: new client doc on readthedocs

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

pep-0458.txt

@@ -85,8 +85,7 @@ to install or update projects from PyPI with TUF metadata.   Package managers
 interested in adopting TUF on the client side may consult its `library
 documentation`__, which was created for this purpose.
 
-__ https://github.com/theupdateframework/tuf/tree/v0.11.1/tuf/client#updaterpy
-
+__ https://theupdateframework.readthedocs.io/en/stable/api/tuf.ngclient.html
 
 Non-goals
 =========
@@ -373,11 +372,11 @@ from PyPI. TUF downloads them and checks them against the TUF metadata that it
 also downloads from the repository. If the downloaded target files are
 trustworthy, TUF then hands them over to the package manager.
 
-The `Metadata`__ document provides information about each type of required
-metadata and its expected content.  The next section covers the different
-kinds of metadata RECOMMENDED for PyPI.
+The `Document formats`__ section of the TUF specification provides information
+about each type of required metadata and its expected content.  The next
+section covers the different kinds of metadata RECOMMENDED for PyPI.
 
-__ https://github.com/theupdateframework/tuf/blob/v0.11.1/docs/METADATA.md
+__ https://theupdateframework.github.io/specification/latest/#document-formats
 
 In addition, all target files SHOULD be available on disk at least two times.
 Once under their original filename, to provide backwards compatibility, and
@@ -509,11 +508,12 @@ that PyPI uses to produce snapshots that can safely coexist and be deleted
 independent of other snapshots [18]_.
 
 Every year, PyPI administrators SHOULD sign for *root* and *targets* role keys.
-Automation will continuously sign for a timestamped snapshot of all projects.
-A `repository management`__ tool is available that can sign metadata files,
-generate cryptographic keys, and manage a TUF repository.
+Automation will continuously sign for a timestamped snapshot of all projects. A
+repository `Metadata API`__ is available that can be used to `manage a TUF
+repository`__.
 
-__ https://github.com/theupdateframework/tuf/blob/v0.11.1/docs/TUTORIAL.md#how-to-create-and-modify-a-tuf-repository
+__ https://theupdateframework.readthedocs.io/en/stable/api/tuf.api.html
+__ https://github.com/theupdateframework/python-tuf/blob/v0.20.0/examples/repo_example/basic_repo.py
 
 In standard operation, the *bin-n* metadata will be updated and signed as new
 distributions are uploaded to PyPI. However, there will also need to be a
@@ -611,14 +611,14 @@ grow correspondingly.  For example, consider the *bins* role.  In August 2013,
 it was found that the size of the *bins* metadata was about 42MB if the *bins*
 role itself signed for about 220K PyPI targets (which are simple indices and
 distributions).  This PEP does not delve into the details, but TUF features a
-so-called "`lazy bin walk`__" scheme that splits a large targets metadata file
+so-called `"hashed bin delegation"`__ scheme that splits a large targets metadata file
 into many small ones.  This allows a TUF client updater to intelligently
 download only a small number of TUF metadata files in order to update any
 project signed for by the *bins* role.  For example, applying this scheme to
 the previous repository resulted in pip downloading between 1.3KB and 111KB to
 install or upgrade a PyPI project via TUF.
 
-__ https://github.com/theupdateframework/tuf/blob/v0.11.1/docs/TUTORIAL.md#delegate-to-hashed-bins
+__ https://github.com/theupdateframework/python-tuf/blob/v0.20.0/examples/repo_example/hashed_bin_delegation.py
 
 Based on our findings as of the time this document was updated for
 implementation (Nov 7 2019), summarized in Tables 2-3, PyPI SHOULD