Enable mypy type checking for pipenv's own code

[matteius]

Closes #5335.

What

• Adds a mirrors-mypy (v2.4.0) pre-commit hook. CI's lint job already runs pre-commit run --all-files, so mypy now gates CI without a workflow change.
• Updates the stale [tool.mypy] config. It was targeting Python 3.7, pointing at a typeshed/ dir that doesn't exist and generating an HTML report that needs lxml. It now targets 3.10, checks pipenv/, excludes pipenv/vendor/ and pipenv/patched/, and uses follow_imports = "skip" so imports into them aren't followed.
• Vendored and patched packages are untouched.
• Fixes all 201 existing errors across pipenv's 77 modules. Most fixes correct annotations or convert legacy # type: comments to inline annotations. Ruff doesn't count type comments as using an import, so they caused unused-import errors once the typing names were imported. # type: ignore is limited to code that's genuinely dynamic, such as the private _normalized_name/_path attributes on importlib.metadata distributions and the latest_version attribute set on them at runtime.

Small real fixes mypy surfaced

• get_pip_args passed a bare src_dir string to list.extend, which would add one argument per character. It now produces ["--src", src_dir]. No caller passes it today.
• Lockfile.from_data built a ProjectFile with a str location, which would break ProjectFile.write() (str has no .open). It now passes a Path.
• Environment.is_satisfied and safe_import used pkg_resources APIs (.location, .has_metadata, .project_name, .activate) on stdlib distributions. Neither path can actually run, so this isn't a user-facing fix. In is_satisfied, the earlier req.specifier check always returns first. The safe_import fallback runs only when _modules[name] is falsy, which never happens. I switched the first to the importlib equivalents and removed the second.

Follow-ups (not in this PR)

Mypy still skips the bodies of functions with no annotations. Turning on check_untyped_defs to check them is the natural next step, best done one module at a time.

Testing

• mypy: no issues in 77 source files
• pre-commit run --all-files: all hooks pass
• Unit: 1760 passed, 10 skipped
• Integration: 250 passed. 15 failed locally, and the same 15 fail on main in this environment (local mirror/index fixtures), so this branch doesn't cause them.

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