Modernize python versions

[tseaver]

Add support for Python 3.11 and 3.12.

Drop support for Python < 3.8, including all Python2 compatibility shims.

Repair bitrot in doctests, and ensure that they run under pytest.

[pfw]

Adding py313 to the tox environment list works now - installing ZODB and dependencies on 3.13 failed last I checked. I think it was just cffi which as of v1.17.0rc1 works.

[pfw]

I've been running with this branch and all is going well for us, here is a small diff to add python3.13 and pypy3.10

diff --git a/tox.ini b/tox.ini
index 0688d55..06dc174 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,15 +1,16 @@
 [tox]
 envlist = 
-    py38,py39,py310,py310-pure,py311,py312,pypy38,pypy39,coverage,docs
+    py38,py39,py310,py310-pure,py311,py312,py313,pypy38,pypy39,pypy310,coverage,docs
 
 [testenv]
 setenv =
     pure: PURE_PYTHON=1
-    !pure-!pypy38-!pypy39: PURE_PYTHON=0
+    !pure-!pypy38-!pypy39-!pypy310: PURE_PYTHON=0
 # The repr of an LFSet is different in pure python
     pure: PYTEST_ADDOPTS=--ignore=hypatia/field/README.txt
     pypy38: PYTEST_ADDOPTS=--ignore=hypatia/field/README.txt
     pypy39: PYTEST_ADDOPTS=--ignore=hypatia/field/README.txt
+    pypy310: PYTEST_ADDOPTS=--ignore=hypatia/field/README.txt
 use_develop = False
 commands = 
     pip install -e .[testing]