Issue title: pip does not pick up pp3-none-any wheels on pypy
 Issue body: **Environment**

* pip version: 20.1.1 (19.3.1 is the last working version)
* Python version: pypy3.6.9-7.3.1
* OS: Arch Linux

**Description**
It appears that `pip` does not consider `pp3-none-any` a valid wheel tag on PyPy 3 after `pip==19.3.1`. This is a problem for [the PEP 615 backport](https://github.com/pganssle/zoneinfo), since it has both a C and a pure Python implementation, and I want to ship a wheel containing only the pure Python implementation *only for PyPy* (anyone else who doesn't have a wheel should fall back to the sdist).

Here is a minimal reproducing bash script, which downloads an arbitrary universal wheel and renames it to end with `pp3-none-any.whl`:

```bash
cd $(mktemp -d)
pypy3 -m venv venv
source venv/bin/activate
pip install -U pip

pip --version
# pip 20.1.1 from /tmp/tmp.IV5fhdUrQt/venv/site-packages/pip (python 3.6)

python --version
# Python 3.6.9 (2ad108f17bdb, Apr 07 2020, 02:59:05)
# [PyPy 7.3.1 with GCC 7.3.1 20180303 (Red Hat 7.3.1-5)]

pip wheel -w wheelhouse tzdata==2020.1
mv wheelhouse/tzdata-2020.1-py2.py3-none-any.whl\
   wheelhouse/tzdata-2020.1-pp3-none-any.whl
pip install -t target --no-index -f wheelhouse tzdata
# Looking in links: wheelhouse
# ERROR: Could not find a version that satisfies the requirement tzdata (from versions: none)
# ERROR: No matching distribution found for tzdata
```

**Expected behavior**
The expected behavior is to install the `pp3` wheel when on PyPy, which was the behavior in 19.3.1:

```
$ pip --version
pip 19.3.1 from /tmp/tmp.IV5fhdUrQt/venv/site-packages/pip (python 3.6)
$ pip install -t target --no-index -f wheelhouse tzdata
Looking in links: wheelhouse
Processing ./wheelhouse/tzdata-2020.1-pp3-none-any.whl
Installing collected packages: tzdata
Successfully installed tzdata-2020.1
```

It's possible that this is another manifestation of #7629. 
 Comments: 
 Comment 0: Hm, this was apparently [done deliberately](https://github.com/pypa/packaging/issues/184), based on the idea that `pp3-none-any` would be a nonsensical tag, but I'm not sure what I should use in this case. I'd be fine with `pp-none-any.whl` here as well, since `python_requires` will prevent it being picked up on PyPy 2.7.

@brettcannon Is this use case, where I have a PyPy-specific build not using any version-specific features of PyPy, something you considered already? Is there a workaround? If not, I think it would be a good idea to add support for at least `pp`, if not `pp3`. I'd really rather not have to build separate, identical wheels for every version of PyPy that's released just so I can tag them with `pp36`, `pp37`, `pp38`, etc.

(P.S. We can probably move this issue over to pypa/packaging, since it seems this was a deliberate choice over there.)
 Comment 1: They are both caused by pypa/packaging#184, but I feel each tag should be discussed separately. `pp272` (#7629) makes less sense to me than `pp2` (this issue).
 Comment 2: I'd bundle them together TBH -- #7629 is the right place within pip for discussing the PyPy tags w/ `packaging.tags` situation.
{'source': 'GithubIssues', 'repo_name_with_owner': 'pypa/pip', 'issue_id': 626549171, 'repo_stars': 9333.0}