Guard TOML config read when tomli/tomllib is unavailable (NameError on Python < 3.11)

[fudianchn]

Problem

configuration.py imports tomllib (or tomli as tomllib) under contextlib.suppress(ImportError), then calls tomllib.load unconditionally. On Python < 3.11 without the tomli backport, the import fails silently, leaving tomllib unbound, so any run in a project with a pyproject.toml crashes (#368):

config = tomllib.load(f)
NameError: name 'tomllib' is not defined

Python < 3.11 is a supported version (pyproject.toml declares python = "^3.10"). 1.7.7 guarded this with a TOMLI_INSTALLED check; the 1.7.8 refactor dropped it.

Fix

Use a try/except that binds tomllib = None when neither the stdlib tomllib nor the tomli backport is importable, and skip reading the TOML configuration in that case — matching 1.7.7, which ran cleanly on the same interpreter — instead of crashing.

-with contextlib.suppress(ImportError):
+try:
     if sys.version_info >= (3, 11):
         import tomllib
     else:
         import tomli as tomllib
+except ImportError:
+    tomllib = None

     def _do_read_toml_configuration(self) -> None:
         """Load configuration information from a *.toml file."""
+        if tomllib is None:
+            return
         with open(self.config_file, "rb") as f:
             config = tomllib.load(f)

Also drops the now-unused contextlib import. When tomllib is available (Python ≥ 3.11, or < 3.11 with tomli), behaviour is unchanged.

Verification

Reproduced in docker python:3.10 (no stdlib tomllib, no tomli installed) with a pyproject.toml containing [tool.docformatter]:

WITHOUT the fix:  NameError: name 'tomllib' is not defined   (exit 1)
WITH    the fix:  docformatter runs cleanly                   (exit 0)

black --check passes on the changed file.

Closes #368