Prevent pip install from trying to compile typed-ast under pypy3

[webknjaz]

What do these changes do?

$sbj

Are there changes in behavior for the user?

N/A

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[webknjaz]

Looks like we need to separate requirements files for pypy vs cpython.

[derlih]

@webknjaz check #5505
When the legacy pip resolver is used, everything installs fine for pypy and cpython

[webknjaz]

We should adopt this to work with any resolver.