PEP 3131: Fix links (#4263)

* PEP 3131: Replace 404 with Internet Archive link

* PEP 3131: Use https

* PEP 3131: Move references inline

peps/pep-3131.rst

@@ -57,8 +57,9 @@ an additional policy is necessary, anyway.
 Specification of Language Changes
 =================================
 
-The syntax of identifiers in Python will be based on the Unicode standard annex
-UAX-31 [1]_, with elaboration and changes as defined below.
+The syntax of identifiers in Python will be based on the `Unicode standard annex
+UAX-31 <https://www.unicode.org/reports/tr31/>`__, with elaboration and changes
+as defined below.
 
 Within the ASCII range (U+0001..U+007F), the valid characters for identifiers
 are the same as in Python 2.5.  This specification only introduces additional
@@ -69,9 +70,10 @@ the ``unicodedata`` module.
 The identifier syntax is ``<XID_Start> <XID_Continue>*``.
 
 The exact specification of what characters have the XID_Start or
-XID_Continue properties can be found in the DerivedCoreProperties
-file of the Unicode data in use by Python (4.1 at the time this
-PEP was written), see [6]_. For reference, the construction rules
+XID_Continue properties can be found in the `DerivedCoreProperties
+file <https://www.unicode.org/Public/4.1.0/ucd/DerivedCoreProperties.txt>`__
+of the Unicode data in use by Python (4.1 at the time this
+PEP was written). For reference, the construction rules
 for these sets are given below. The XID_* properties are derived
 from ID_Start/ID_Continue, which are derived themselves.
 
@@ -94,7 +96,7 @@ comparison of identifiers is based on NFKC.
 
 A non-normative HTML file listing all valid identifier characters for
 Unicode 4.1 can be found at
-http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.
+https://web.archive.org/web/20081016132748/http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.
 
 Policy Specification
 ====================
@@ -136,8 +138,9 @@ The following changes will need to be made to the parser:
 Open Issues
 ===========
 
-John Nagle suggested consideration of Unicode Technical Standard #39,
-[2]_, which discusses security mechanisms for Unicode identifiers.
+John Nagle suggested consideration of `Unicode Technical Standard #39
+<https://www.unicode.org/reports/tr39/>`__,
+which discusses security mechanisms for Unicode identifiers.
 It's not clear how that can precisely apply to this PEP; possible
 consequences are
 
@@ -153,7 +156,8 @@ needs two identifiers to compare them for confusion - is it possible
 to somehow apply it to a single identifier only, and warn?
 
 In follow-up discussion, it turns out that John Nagle actually
-meant to suggest UTR#36, level "Highly Restrictive", [3]_.
+meant to suggest `UTR#36 <https://www.unicode.org/reports/tr36/>`__,
+level "Highly Restrictive".
 
 Several people suggested to allow and ignore formatting control
 characters (general category Cf), as is done in Java, JavaScript, and
@@ -164,15 +168,17 @@ later.
 Some people would like to see an option on selecting support
 for this PEP at run-time; opinions vary on what precisely
 that option should be, and what precisely its default value
-should be. Guido van Rossum commented in [5]_ that a global
-flag passed to the interpreter is not acceptable, as it would
+should be. `Guido van Rossum commented
+<https://mail.python.org/pipermail/python-3000/2007-May/007925.html>`__
+that a global flag passed to the interpreter is not acceptable, as it would
 apply to all modules.
 
 Discussion
 ==========
 
-Ka-Ping Yee summarizes discussion and further objection
-in [4]_ as such:
+`Ka-Ping Yee summarizes discussion and further objection
+<https://mail.python.org/pipermail/python-3000/2007-June/008161.html>`__
+as such:
 
 A. Should identifiers be allowed to contain any Unicode letter?
 
@@ -250,16 +256,6 @@ F.  Which normalization form should be used, NFC or NFKC?
 G.  Should source code be required to be in normalized form?
 
 
-References
-==========
-
-.. [1] http://www.unicode.org/reports/tr31/
-.. [2] http://www.unicode.org/reports/tr39/
-.. [3] http://www.unicode.org/reports/tr36/
-.. [4] https://mail.python.org/pipermail/python-3000/2007-June/008161.html
-.. [5] https://mail.python.org/pipermail/python-3000/2007-May/007925.html
-.. [6] http://www.unicode.org/Public/4.1.0/ucd/DerivedCoreProperties.txt
-
 Copyright
 =========