Enforce platform-specific CPython version requirements for Windows in tox

This also changes the black and isort tests to use CPython 3.9
because Cygwin currently doesn't have CPython 3.10 available.

tests/test_clean.py

@@ -3,21 +3,6 @@ import sys
 
 import pytest
 
-# Python 2.7 on Windows does not have an `os.symlink()` function.
-# PyPy on Windows raises NotImplementedError when `os.symlink()` is called.
-# Older Python 3 versions on Windows require admin rights to create symlinks.
-#
-# In addition, functions like `os.path.realpath()` on Windows Pythons < 3.8
-# do not resolve symlinks and directory junctions correctly,
-# and `shutil.rmtree()` will fail to delete directory junctions.
-#
-# For these reasons, if the tests are running on Windows with Python < 3.8
-# or with PyPy, the entire link test suite must be skipped.
-#
-if sys.platform[:5] == "win32" and (sys.version_info < (3, 8) or "pypy" in sys.version.lower()):
-    reason = "It is impossible to perform link tests on this platform"
-    pytestmark = pytest.mark.skip(reason=reason)
-
 
 def test_clean_default(root, home, dotfiles, run_dotbot):
     """Verify clean uses default unless overridden."""

tests/test_link.py

@@ -3,21 +3,6 @@ import sys
 
 import pytest
 
-# Python 2.7 on Windows does not have an `os.symlink()` function.
-# PyPy on Windows raises NotImplementedError when `os.symlink()` is called.
-# Older Python 3 versions on Windows require admin rights to create symlinks.
-#
-# In addition, functions like `os.path.realpath()` on Windows Pythons < 3.8
-# do not resolve symlinks and directory junctions correctly,
-# and `shutil.rmtree()` will fail to delete directory junctions.
-#
-# For these reasons, if the tests are running on Windows with Python < 3.8
-# or with PyPy, the entire link test suite must be skipped.
-#
-if sys.platform[:5] == "win32" and (sys.version_info < (3, 8) or "pypy" in sys.version.lower()):
-    reason = "It is impossible to perform link tests on this platform"
-    pytestmark = pytest.mark.skip(reason=reason)
-
 
 def test_link_canonicalization(home, dotfiles, run_dotbot):
     """Verify links to symlinked destinations are canonical.

tox.ini

@@ -1,13 +1,20 @@
 [tox]
+; On Windows, only CPython >= 3.8 is supported.
+; All older versions, and PyPy, lack full symlink support.
 envlist =
-    py{27, 35, 36, 37, 38, 39, 310}
+    py{38, 39, 310}-all_platforms
-    pypy{2, 3}
+    py{27, 35, 36, 37}-most_platforms
-    py310-black
+    pypy{2, 3}-most_platforms
-    py310-isort
+    py39-black
+    py39-isort
 skip_missing_interpreters = true
 
 
 [testenv]
+platform =
+    all_platforms: cygwin|darwin|linux|win32
+    most_platforms: cygwin|darwin|linux
+
 deps =
     pytest
     pytest-randomly
@@ -18,7 +25,7 @@ commands =
     pytest tests/
 
 
-[testenv:py310-black]
+[testenv:py39-black]
 skip_install = true
 skip_build = true
 deps =
@@ -28,7 +35,7 @@ commands =
     black --check dotbot/ tests/
 
 
-[testenv:py310-isort]
+[testenv:py39-isort]
 skip_install = true
 skip_build = true
 deps =