Drop *_proxy from passenv

For tox 3.x and earlier, passenv was a space-separated list; as of tox
4.0.0, it's comma-separated. For a while, our spaces would be silently
included in the now-one-and-only passenv value parsed (which wasn't
great, but mostly just caused confusion) -- as of tox 4.0.6, however, it
became a hard error, and all tests would fail like

  pass_env values cannot contain whitespace, use comma to have multiple
  values in a single line, invalid values found 'SWIFT_* *_proxy'

Unfortunately, we don't really know what versions of tox all our various
stakeholders might want/need to use (though we currently set a
minversion of 3.18.0), and (to my knowledge) there's no way to specify
multiple values for passenv in a way that would be compatible with both
old and new tox.

Fortunately, as of 3.14.4, tox include HTTP_PROXY, HTTPS_PROXY, and
NO_PROXY in passenv by default. I'm fairly certain those were the only
reason we had *_proxy to begin with, so just drop it.

Change-Id: I8ddfbc6376434c39e231846135a99bd0bdca9497

.zuul.yaml

@@ -10,6 +10,9 @@
     # for installation.
     required-projects:
       - opendev.org/openstack/python-swiftclient
+    vars:
+      # New tox keeps breaking things as of 2023-01
+      ensure_tox_version: '<4'
 
 - job:
     name: swiftclient-functional

tox.ini

@@ -18,7 +18,7 @@ commands = sh -c '(find . -not \( -type d -name .?\* -prune \) \
            -print0) | xargs -0 rm -rf'
            stestr run {posargs}
 allowlist_externals = sh
-passenv = SWIFT_* *_proxy
+passenv = SWIFT_*
 
 [testenv:pep8]
 commands =