tox: Set --basetemp to work around MAX_PATH limitation on Windows

tox.ini

@@ -1,15 +1,33 @@
 [tox]
-envlist = py37,py38,pypy3,pre-commit
+# Platform specification support is available since version 2.0, see:
+# https://tox.wiki/en/latest/example/platform.html#basic-multi-platform-example
+minversion = 2.0
+envlist = {py37,py38,pypy3}-{unix,windows},pre-commit
 
 [testenv]
 deps = -rrequirements-dev.txt
 passenv = APPDATA HOME LOCALAPPDATA PROGRAMFILES RUSTUP_HOME
+platform =
+    unix: darwin|cygwin|linux\d*|aix\d*|sunos\d*|freebsd\d*
+    windows: win32
+# It's necessary to override the `envtmpdir` because the default one breaks
+# some tests. Those tests depend on being run in a directory that is not inside
+# a git repository, and the `envtmpdir` defaults to
+# `{toxinidir}/.tox/{envname}/tmp` where `toxinidir` also contains a `.git`
+# directory.
+envtmpdir =
+    unix: /tmp/pre-commit-{envname}
+    windows: C:\tmp\pre-commit-{envname}
 commands =
     coverage erase
-    coverage run -m pytest {posargs:tests}
+    coverage run -m pytest --basetemp="{envtmpdir}" {posargs:tests}
     coverage report
 
 [testenv:pre-commit]
+# We need to set `envtmpdir` because it was set for the `testenv` above. If
+# this is omitted, envtmpdir is empty and the whole git repository is deleted
+# by tox.
+envtmpdir = {envdir}/.tmp
 skip_install = true
 deps = pre-commit
 commands = pre-commit run --all-files --show-diff-on-failure