stubtest_stdlib.py: suppress output from pip

[AlexWaygood]

Following #9734, there's now some fairly noisy output from pip if you run stubtest_stdlib.py locally, which I do quite a lot. This makes the output of the script hard to read. This PR suppresses the output from pip.

Prior to this PR:

(.venv) C:\Users\alexw\coding\typeshed>python tests/stubtest_stdlib.py
Collecting mypy==1.0
  Using cached mypy-1.0.0-cp311-cp311-win_amd64.whl (8.8 MB)
Collecting typing-extensions>=3.10
  Using cached typing_extensions-4.5.0-py3-none-any.whl (27 kB)
Collecting mypy-extensions>=0.4.3
  Using cached mypy_extensions-1.0.0-py3-none-any.whl (4.7 kB)
Installing collected packages: typing-extensions, mypy-extensions, mypy
Successfully installed mypy-1.0.0 mypy-extensions-1.0.0 typing-extensions-4.5.0

[notice] A new release of pip available: 22.3.1 -> 23.0.1
[notice] To update, run: C:\Users\alexw\AppData\Local\Temp\tmp86thq0va\Scripts\python.exe -m pip install --upgrade pip
Found existing installation: setuptools 65.5.0
Uninstalling setuptools-65.5.0:
  Successfully uninstalled setuptools-65.5.0
C:\Users\alexw\AppData\Local\Temp\tmp86thq0va\Scripts\python.exe -m mypy.stubtest --check-typeshed --custom-typeshed-dir . --allowlist tests\stubtest_allowlists\py3_common.txt --allowlist tests\stubtest_allowlists\py311.txt --allowlist tests\stubtest_allowlists\win32.txt --allowlist tests\stubtest_allowlists\win32-py311.txt
Success: no issues found in 484 modules
stubtest succeeded

With this PR applied:

(.venv) C:\Users\alexw\coding\typeshed>python tests/stubtest_stdlib.py
Setting up an isolated virtual environment...
C:\Users\alexw\AppData\Local\Temp\tmp1bkull8d\Scripts\python.exe -m mypy.stubtest --check-typeshed --custom-typeshed-dir . --allowlist tests\stubtest_allowlists\py3_common.txt --allowlist tests\stubtest_allowlists\py311.txt --allowlist tests\stubtest_allowlists\win32.txt --allowlist tests\stubtest_allowlists\win32-py311.txt
Success: no issues found in 484 modules
stubtest succeeded