contrib: freeze_packages should pin latest pip/etc if possible
contrib: freeze_packages should pin latest pip/etc if possible

We've already been pinning pip/setuptools/wheel; it is `pip freeze --all`
(the "--all" arg) that does that. This change just tries to ensure that
we pin the latest version when possible.
Previously if e.g. "pip" was not pulled in by any package in "requirements${i}.txt",
we would just pin whatever is installed locally.

spesmilo/electrum,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,pooler/electrum-ltc,wakiyamap/electrum-mona