fix: polyfill nullcontext for py3.6. Document compatibility with Pyth…

…on versions.

README.rst

@@ -153,3 +153,10 @@ The code is dual-licenced under both
 - `Apache License, Version 2.0 <https://choosealicense.com/licenses/apache-2.0>`_
 
 at your option.
+
+Supported Python versions
+-------------------------
+
+The versions that are regularly tested can be found `here <https://github.com/bgeron/diff-pdf-visually/blob/main/tox.ini>`_, that's probably Python 3.8 and Python 3.9.
+
+For your convenience we declare more Python versions acceptable in ``pyproject.toml``, but the non-tested versions could potentially break from time to time. My goal is to support basically Python 3.x; please let me know if something doesn't work on an older version.

diff_pdf_visually/__init__.py

@@ -1,5 +1,5 @@
 """
-'Quickly check whether there is a visible difference between two PDFs, using ImageMagick and pdftocairo.
+Quickly check whether there is a visible difference between two PDFs, using ImageMagick and pdftocairo.
 """
 
 __version__ = '1.6.1'

diff_pdf_visually/diff.py

@@ -9,7 +9,7 @@
 
 import os.path, pathlib, subprocess, sys, tempfile, time
 from concurrent.futures import ThreadPoolExecutor
-from contextlib import nullcontext
+from .polyfill import nullcontext
 from .constants import DEFAULT_THRESHOLD, DEFAULT_VERBOSITY, DEFAULT_DPI
 from .constants import VERB_PRINT_REASON, VERB_PRINT_TMPDIR
 from .constants import VERB_PERPAGE, VERB_PRINT_CMD, VERB_ROUGH_PROGRESS

diff_pdf_visually/polyfill.py

@@ -0,0 +1,14 @@
+"""
+Reimplementations of standard functionality to get better compatibility with older
+Python versions.
+"""
+
+class nullcontext:
+    def __init__(self, enter_result):
+        self._enter_result = enter_result
+
+    def __enter__(self):
+        return self._enter_result
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        pass