diff --git a/README.md b/README.md
index 60a6b8b..047b9c5 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,14 @@ automatically by Travis-CI)
     pip install pytest
     python -m pytest
 
+To run the full matrix of Python versions locally, use two commands:
+
+    # old Python versions (via Docker)
+    docker run --rm -v .:/src -w /src makukha/multipython:unsafe tox run -m old
+
+    # modern Python versions (via uv)
+    uvx --with tox-uv tox run -m new
+
 Quick Start
 -----------
 
diff --git a/mf2util.py b/mf2util.py
index 6354d0c..b0015f6 100644
--- a/mf2util.py
+++ b/mf2util.py
@@ -385,7 +385,7 @@ def do_convert(match):
         for tagname, attributes in URL_ATTRIBUTES.items():
             for attribute in attributes:
                 pattern = re.compile(
-                    '<%s[^>]*?%s\s*=\s*[\'"](.*?)[\'"]' % (tagname, attribute),
+                    r'<%s[^>]*?%s\s*=\s*[\'"](.*?)[\'"]' % (tagname, attribute),
                     flags=re.DOTALL | re.MULTILINE | re.IGNORECASE)
                 html = pattern.sub(do_convert, html)
 
@@ -484,10 +484,10 @@ def parse_datetime(s):
     if not s:
         return None
 
-    s = re.sub('\s+', ' ', s)
-    date_re = "(?P<year>\d{4,})-(?P<month>\d{1,2})-(?P<day>\d{1,2})"
-    time_re = "(?P<hour>\d{1,2}):(?P<minute>\d{2})(:(?P<second>\d{2})(\.(?P<microsecond>\d+))?)?"
-    tz_re = "(?P<tzz>Z)|(?P<tzsign>[+-])(?P<tzhour>\d{1,2}):?(?P<tzminute>\d{2})"
+    s = re.sub(r'\s+', ' ', s)
+    date_re = r"(?P<year>\d{4,})-(?P<month>\d{1,2})-(?P<day>\d{1,2})"
+    time_re = r"(?P<hour>\d{1,2}):(?P<minute>\d{2})(:(?P<second>\d{2})(\.(?P<microsecond>\d+))?)?"
+    tz_re = r"(?P<tzz>Z)|(?P<tzsign>[+-])(?P<tzhour>\d{1,2}):?(?P<tzminute>\d{2})"
     dt_re = "%s((T| )%s ?(%s)?)?$" % (date_re, time_re, tz_re)
 
     m = re.match(dt_re, s)
diff --git a/setup.py b/setup.py
index 5dd522b..fe830f4 100644
--- a/setup.py
+++ b/setup.py
@@ -24,7 +24,7 @@ def run_tests(self):
 
 
 setup(name='mf2util',
-      version='0.5.2',
+      version='0.5.2+zegnat.1',
       description='Python Microformats2 utilities, a companion to mf2py',
       long_description="""
 Microformats2 Utilities
diff --git a/tests/test_interpret.py b/tests/test_interpret.py
index e134d94..19b32d2 100644
--- a/tests/test_interpret.py
+++ b/tests/test_interpret.py
@@ -4,12 +4,13 @@
 """
 from __future__ import unicode_literals
 from datetime import datetime, date, timedelta
+import io
 import mf2util
 import json
 
 
 def load_test(testname):
-    return json.load(open('tests/interpret/%s.json' % testname))
+    return json.load(io.open('tests/interpret/%s.json' % testname, encoding='utf-8'))
 
 
 def test_event():
diff --git a/tests/test_post_type_discovery.py b/tests/test_post_type_discovery.py
index fffdd54..a169fbe 100644
--- a/tests/test_post_type_discovery.py
+++ b/tests/test_post_type_discovery.py
@@ -1,6 +1,7 @@
 """Tests for post_type_discovery
 """
 
+import io
 import json
 import mf2util
 
@@ -21,7 +22,7 @@ def test_post_type_discovery():
             ('posttype/hcard_org', 'org'),
             # TODO add more tests
     ]:
-        parsed = json.load(open('tests/' + test + '.json'))
+        parsed = json.load(io.open('tests/' + test + '.json', encoding='utf-8'))
         types = (['h-card'] if implied_type in ('person', 'org')
                  else ['h-entry', 'h-event'])
         entry = mf2util.find_first_entry(parsed, types)
diff --git a/tox.ini b/tox.ini
new file mode 100644
index 0000000..938071e
--- /dev/null
+++ b/tox.ini
@@ -0,0 +1,11 @@
+[tox]
+envlist = py27,py35,py36,py37,py38,py39,py310,py311,py312,py313,py314
+labels =
+    old = py27,py35,py36,py37
+    new = py38,py39,py310,py311,py312,py313,py314
+
+[testenv]
+deps =
+    pytest
+    mf2py
+commands = pytest tests