test: exec the codespell executable consistently

Currently, in test_basic.py, the run_codespell and run_codespell_stdin
functions exec the codespell script from PATH, not the one from the
current directory.  This means that the codespell_lib package MUST first
be installed, before running pytest.

The current behavior is unsafe, since an user running pytest in the
global environment may get an error or, worse, may actually try to test
an old version.  Additionally this behavior is not documented in
README.md.

Update the run_codespell and run_codespell_stdin functions to exec the
codespell script via `python -m codespell_lib`.  This change will ensure
that python will try to search the __main__ module from the current
directory first.  Copy the codespell_lib directory to the cwd directory,
and configure codespell to ignore it using the `-S` option, in order to
make the environment clean.

Ensure that the cwd parameter in run_codespell and run_codespell_stdin
is never None, since the codespell script must not be executed from the
current directory.  For consistency, also make the args parameter
required.

codespell_lib/tests/test_basic.py

@@ -7,7 +7,7 @@
 import sys
 from io import StringIO
 from pathlib import Path
-from shutil import copyfile
+from shutil import copyfile, copytree
 from typing import Any, Generator, Optional, Tuple, Union
 
 import pytest
@@ -64,13 +64,15 @@ def main(
 
 
 def run_codespell(
-    args: Tuple[Any, ...] = (),
-    cwd: Optional[Path] = None,
+    args: Tuple[Any, ...],
+    cwd: Path,
 ) -> int:
     """Run codespell."""
+    lib = "codespell_lib"
+    copytree(lib, cwd / lib, dirs_exist_ok=True)
     args = tuple(str(arg) for arg in args)
     proc = subprocess.run(
-        ["codespell", "--count", *args],  # noqa: S603, S607
+        [sys.executable, "-m", lib, "-S", lib, "--count", *args],  # noqa: S603, S607
         cwd=cwd,
         capture_output=True,
         encoding="utf-8",
@@ -83,9 +85,9 @@ def run_codespell(
 def test_command(tmp_path: Path) -> None:
     """Test running the codespell executable."""
     # With no arguments does "."
-    assert run_codespell(cwd=tmp_path) == 0
+    assert run_codespell(args=(), cwd=tmp_path) == 0
     (tmp_path / "bad.txt").write_text("abandonned\nAbandonned\nABANDONNED\nAbAnDoNnEd")
-    assert run_codespell(cwd=tmp_path) == 4
+    assert run_codespell(args=(), cwd=tmp_path) == 4
 
 
 def test_basic(
@@ -1196,11 +1198,13 @@ def FakeStdin(text: str) -> Generator[None, None, None]:
 def run_codespell_stdin(
     text: str,
     args: Tuple[Any, ...],
-    cwd: Optional[Path] = None,
+    cwd: Path,
 ) -> int:
     """Run codespell in stdin mode and return number of lines in output."""
+    lib = "codespell_lib"
+    copytree(lib, cwd / lib, dirs_exist_ok=True)
     proc = subprocess.run(
-        ["codespell", *args, "-"],  # noqa: S603, S607
+        [sys.executable, "-m", lib, "-S", lib, *args, "-"],  # noqa: S603, S607
         cwd=cwd,
         input=text,
         capture_output=True,