modernize: use Python 3.8+ features

Follow-up to the py39-modernize work, covering features available
since Python 3.8 (the minimum supported version is 3.11):

- use the walrus operator (PEP 572) for assignment expressions where
  it removes a separate assign-then-test line and reads more clearly:
  - version.parse_version: inline the re.match result into the test
  - nanorst.rst_to_text: while char := text.read(1)
  - cockpit.runner read_stream: while line := await stream.readline()
  - tar_cmds._import_tar: while tarinfo := tar.next()

Most other Python 3.8 features are already in use (shlex.join, bare
@lru_cache, typing.Protocol/Literal), so this pass is small.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/borg/archiver/tar_cmds.py

@@ -293,10 +293,7 @@ class TarMixIn:
 
         tar = tarfile.open(fileobj=tarstream, mode="r|", ignore_zeros=args.ignore_zeros)
 
-        while True:
-            tarinfo = tar.next()
-            if not tarinfo:
-                break
+        while tarinfo := tar.next():
             if tarinfo.isreg():
                 status = tfo.process_file(tarinfo=tarinfo, status="A", type=stat.S_IFREG, tar=tar)
                 archive.stats.nfiles += 1

src/borg/cockpit/runner.py

@@ -44,10 +44,7 @@ class BorgRunner:
             )
 
             async def read_stream(stream, stream_name):
-                while True:
-                    line = await stream.readline()
-                    if not line:
-                        break
+                while line := await stream.readline():
                     decoded_line = line.decode("utf-8", errors="replace").rstrip()
                     if decoded_line:
                         self.log_callback({"type": "stream_line", "stream": stream_name, "line": decoded_line})

src/borg/helpers/nanorst.py

@@ -65,10 +65,7 @@ def rst_to_text(text, state_hook=None, references=None):
 
     inline_single = ("*", "`")
 
-    while True:
-        char = text.read(1)
-        if not char:
-            break
+    while char := text.read(1):
         next = text.peek(1)  # type: str
 
         if state == "text":

src/borg/version.py

@@ -19,8 +19,7 @@ def parse_version(version):
         (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)   # version, e.g. 1.2.33
         (?P<prerelease>\.?(?P<ptype>a|b|rc|dev)(?P<pnum>\d+))?  # optional prerelease, e.g. a1 or b2 or rc33 or .dev1
     """
-    m = re.match(version_re, version, re.VERBOSE)
-    if m is None:
+    if (m := re.match(version_re, version, re.VERBOSE)) is None:
         raise ValueError("Invalid version string %s" % version)
     gd = m.groupdict()
     version = [int(gd["major"]), int(gd["minor"]), int(gd["patch"])]