chore: remove pre python 3.5 regex related workaround (#2042)

Removing a pre-python 3.5 regex related workaround as it uses a private
method from the `re` module, and mypy complains about this, it is also
no longer needed as we no longer support pre 3.5 versions of python.

No tests are added as all changed parts of the `Builtin_REPLACE`
function is already covered by existing tests.

CHANGELOG.md

@@ -90,6 +90,22 @@ and will be removed for release.
 <!-- -->
 <!-- -->
 
+- Removed a pre python 3.5 regex related workaround in the REPLACE SPARQL
+  function.
+  [PR #2042](https://github.com/RDFLib/rdflib/pull/2042).
+
+<!-- -->
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+<!-- CHANGE BARRIER: END -->
+<!-- -->
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+
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+
 - PLACEHOLDER.
   Description of changes.
   Closed [issue #....](https://github.com/RDFLib/rdflib/issues/).

rdflib/plugins/sparql/operators.py

@@ -12,7 +12,6 @@
 import operator as pyop  # python operators
 import random
 import re
-import sys
 import uuid
 import warnings
 from decimal import ROUND_HALF_UP, Decimal, InvalidOperation
@@ -227,28 +226,6 @@ def Builtin_REPLACE(expr, ctx):
     # python uses \1, xpath/sparql uses $1
     replacement = re.sub("\\$([0-9]*)", r"\\\1", replacement)
 
-    def _r(m):
-
-        # Now this is ugly.
-        # Python has a "feature" where unmatched groups return None
-        # then re.sub chokes on this.
-        # see http://bugs.python.org/issue1519638 , fixed and errs in py3.5
-
-        # this works around and hooks into the internal of the re module...
-
-        # the match object is replaced with a wrapper that
-        # returns "" instead of None for unmatched groups
-
-        class _m:
-            def __init__(self, m):
-                self.m = m
-                self.string = m.string
-
-            def group(self, n):
-                return m.group(n) or ""
-
-        return re._expand(pattern, _m(m), replacement)
-
     cFlag = 0
     if flags:
         # Maps XPath REGEX flags (http://www.w3.org/TR/xpath-functions/#flags)
@@ -258,12 +235,8 @@ def group(self, n):
 
         # @@FIXME@@ either datatype OR lang, NOT both
 
-    # this is necessary due to different treatment of unmatched groups in
-    # python versions. see comments above in _r(m).
-    compat_r = str(replacement) if sys.version_info[:2] >= (3, 5) else _r
-
     return Literal(
-        re.sub(str(pattern), compat_r, text, cFlag),
+        re.sub(str(pattern), replacement, text, cFlag),
         datatype=text.datatype,
         lang=text.language,
     )