mypy_test.py: Allow non-types dependencies (#9408)

The approach is pretty similar to the approach I took in #9382 for regr_test.py: dynamically create virtual environments for testing stubs packages in isolation. However, since mypy_test.py tests all of typeshed's stubs in succession (and since lots of typeshed's stubs packages depend on other typeshed stubs packages), the performance issues with creating a virtual environment for testing each stubs package are even more severe than with regr_test.py.

I mitigate the performance issues associated with dynamically creating virtual environments in two ways:

- Dynamically creating venvs is mostly slow due to I/O bottlenecks. Creating the virtual environments can be sped up dramatically by creating them concurrently in a threadpool. The same goes for pip installing the requirements into the venvs -- though unfortunately, we have to use pip with --no-cache-dir, as the pip cache gets easily corrupted if you try to do concurrent reads and writes to the pip cache.

- If types-pycocotools requires numpy>=1 and types-D3DShot also requires numpy>=1 (for example), there's no need to create 2 virtual environments. The same requirements have been specified, so they can share a virtual environment between them. This means we don't have to create nearly so many virtual environments.

tests/utils.py

@@ -91,7 +91,10 @@ def read_dependencies(distribution: str) -> PackageDependencies:
         if maybe_typeshed_dependency in pypi_name_to_typeshed_name_mapping:
             typeshed.append(pypi_name_to_typeshed_name_mapping[maybe_typeshed_dependency])
         else:
-            external.append(dependency)
+            # convert to Requirement and then back to str
+            # to make sure that the requirements all have a normalised string representation
+            # (This will also catch any malformed requirements early)
+            external.append(str(Requirement(dependency)))
     return PackageDependencies(tuple(typeshed), tuple(external))
 
 
@@ -142,7 +145,7 @@ def make_venv(venv_dir: Path) -> VenvInfo:
     try:
         venv.create(venv_dir, with_pip=True, clear=True)
     except subprocess.CalledProcessError as e:
-        if "ensurepip" in e.cmd:
+        if "ensurepip" in e.cmd and b"KeyboardInterrupt" not in e.stdout.splitlines():
             print_error(
                 "stubtest requires a Python installation with ensurepip. "
                 "If on Linux, you may need to install the python3-venv package."