REGR: show_versions
regression in #31660 (no need to backport)

master
```
INSTALLED VERSIONS
------------------
{k:<17}: {stat}
{k:<17}: {stat}
{k:<17}: {stat}
...
```

this PR
```
INSTALLED VERSIONS
------------------
commit           : b11e0647d42a27c279f3c46d5ce26d79bb5f5dec
python           : 3.7.4.final.0
python-bits      : 64
...
```

merits test? @jbrockmendel looks like the fakemodule tests don't clean-up. will look tomorrow if can't get to machine later this evening. could maybe use monkeypatch VERSIONS or just delete fakemodule from VERSIONS after test ok, test here isnt important enough to be a blocker > Just an idea, we could rename `show_versions` to `_get_versions` and make it a generator yielding the version information. Then implement a small `show_versions` function that prints the results of `_get_versions`.

have started #32042 to clean-up since `get_sys_info` was stringifying the `None`s which then appears in the json output. so could incorporate this suggestion there. Thanks @simonjayhawkins 