Provide a mechanism to AND interpreter constraint strings

[Eric-Arellano]

In #678, we changed --interpreter-constraint to OR between multiple values. This is useful for disjoint constraints, e.g. CPython==2.7.* OR CPython>=3.5.

We explained that you can AND by using a single constraint, e.g. CPython>=3.5,<3.6.

However, there is no way to AND with the entire requirement string, e.g. there is no way to say CPython==3.7.* AND CPython==3.8.0 without manually merging the constraints into one.

While this may be fine for most usages, Pants needs to be able to AND the entire requirement string. Specifically, in Pants, constraints should be ANDed accross targets (but ORed within the target). For example, the below results in CPython==3.7.* AND CPython==3.6.*.

python_library(
  name="dep_global",
  source="test_compat.py",
)

python_library(
  name="dep",
  source="test_compat.py",
  compatibility='CPython==3.7.*',
)

python_tests(
  name = 'compat',
  source='test_compat.py',
  dependencies=[":dep", ":dep_global"],
  compatibility='CPython==3.6.*',
)

It's not clear what the API should look like for this. --interpreter-constraint should certainly still OR. Possibly, we introduce a new flag?

Possibly we parametrize whether --interpreter-constraint should OR vs. AND? (Although, we may likely need to support both. It's unclear to me if we can use static analysis to avoid this by having Pants pre-process the constraints.)

[Eric-Arellano]

I think I saw a library last week that understands requirement strings and can do algebra between them, such as merging them. I'm going to first explore if we can use that approach of static analysis before having to resort to this change to Pex.

[Eric-Arellano]

Per the suggestion in #918 (comment), the needed functionality was achieved in Pants by combining requirement strings there to AND where necessary.