Use `pypyX.Y` for PyPy `python-version` input

Description:
Use pypyX.Y rather than pypy-X.Y for python-version input
Still allow pypy-X.Y for backward compatibility, maybe deprecate its usage ?

Justification:
This allows python-version to match an actual executable name (c.f. note 1) and allows nox (& probably tox too) to directly use python-version to narrow environment to be tested in a matrix job.

It matches actual PyPy archive names e.g. pypy3.9-v7.3.8-linux64.tar.bz2, pypy2.7-v7.3.8-linux64.tar.bz2, ...

It would also be more in sync with virtualenv python discovery which states:

{python implementation name}{version}{architecture}
We have the following restrictions:
the python implementation is all alphabetic characters (python means any implementation, and if is missing it defaults to python),
the version is a dot separated version number,
the architecture is either -64 or -32 (missing means any).

The current way to get this result involves removing the - which is not something workflow expressions currently allows & thus involves some bash magic to happen:

jobs:
  test:
    name: ${{ matrix.os }} / ${{ matrix.python_version }}
    runs-on: ${{ matrix.os }}-latest
    strategy:
      fail-fast: false
      matrix:
        os: [Ubuntu, Windows, macOS]
        python_version:
          ["3.7", "3.8", "3.9", "3.10", "pypy-3.7", "pypy-3.8", "pypy-3.9"]
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-python@v3
        with:
          python-version: ${{ matrix.python_version }}
      - run: |
          INTERPRETER_SPEC=${{ matrix.python_version }}
          INTERPRETER_SPEC=${INTERPRETER_SPEC/-/}  # remove '-' in "pypy-3.x" -> "pypy3.x" to match executable name
          pipx run nox --error-on-missing-interpreters -s tests-${INTERPRETER_SPEC}
        shell: bash

It would allow to rewrite the workflow as:

jobs:
  test:
    name: ${{ matrix.os }} / ${{ matrix.python_version }}
    runs-on: ${{ matrix.os }}-latest
    strategy:
      fail-fast: false
      matrix:
        os: [Ubuntu, Windows, macOS]
        python_version:
          ["3.7", "3.8", "3.9", "3.10", "pypy3.7", "pypy3.8", "pypy3.9"]
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-python@v3
        with:
          python-version: ${{ matrix.python_version }}
      - run: pipx run nox --error-on-missing-interpreters -s tests-${{ matrix.python_version }}

Note 1: Windows is missing those executables for 3.7 & 3.8 for now but they will be available in PyPy 7.3.9

Are you willing to submit a PR?

One to create missing symlinks on older versions: Create missing pypyX.Y symlinks #347
One for pypX.Y naming scheme: Use pypyX.Y for PyPy python-version input #349