conformance - subprocess.run to use utf-8 to avoid issues on Windows

It was previously failing only for pyright but added it for all type checkers just to be sure.
```
Traceback (most recent call last):
  File "L:\Projects\Github\typing\conformance\src\main.py", line 260, in <module>
    main()
  File "L:\Projects\Github\typing\conformance\src\main.py", line 253, in main
    run_tests(root_dir, type_checker, test_cases, skip_timing=options.skip_timing)
  File "L:\Projects\Github\typing\conformance\src\main.py", line 34, in run_tests
    type_checker.parse_errors(output.splitlines())
  File "L:\Projects\Github\typing\conformance\src\type_checker.py", line 206, in parse_errors
    assert line.count(":") >= 3, f"Failed to parse line: {line!r}"
           ^^^^^^^^^^^^^^^^^^^^
AssertionError: Failed to parse line: 'Â\xa0Â\xa0Use list[T] to indicate a list type or T1 | T2 to indicate a union type (reportInvalidTypeForm)'
```

Update type_checker.py

Author: Andrej730

conformance/src/type_checker.py

@@ -114,7 +114,7 @@ def run_tests(self, test_files: Sequence[str]) -> dict[str, str]:
             "--enable-error-code",
             "deprecated",
         ]
-        proc = run(command, stdout=PIPE, text=True)
+        proc = run(command, stdout=PIPE, text=True, encoding="utf-8")
         lines = proc.stdout.split("\n")
 
         # Add results to a dictionary keyed by the file name.
@@ -174,7 +174,7 @@ def get_version(self) -> str:
 
     def run_tests(self, test_files: Sequence[str]) -> dict[str, str]:
         command = [sys.executable, "-m", "pyright", ".", "--outputjson"]
-        proc = run(command, stdout=PIPE, text=True)
+        proc = run(command, stdout=PIPE, text=True, encoding="utf-8")
         output_json = json.loads(proc.stdout)
         diagnostics = output_json["generalDiagnostics"]
 
@@ -253,7 +253,7 @@ def get_version(self) -> str:
         return version
 
     def run_tests(self, test_files: Sequence[str]) -> dict[str, str]:
-        proc = run(["pyre", "check"], stdout=PIPE, text=True)
+        proc = run(["pyre", "check"], stdout=PIPE, text=True, encoding="utf-8")
         lines = proc.stdout.split("\n")
 
         # Add results to a dictionary keyed by the file name.