tests: avoid putting build products into source directory (#2353)

* tests: keep source dir clean

* ci: make first build inplace

* ci: drop dev setting (wasn't doing anything)

* tests: warn if source directory is dirty

tests/conftest.py

@@ -13,6 +13,8 @@ import textwrap
 
 import pytest
 
+import env
+
 # Early diagnostic for failed imports
 import pybind11_tests  # noqa: F401
 
@@ -20,6 +22,11 @@ _unicode_marker = re.compile(r'u(\'[^\']*\')')
 _long_marker = re.compile(r'([0-9])L')
 _hexadecimal = re.compile(r'0x[0-9a-fA-F]+')
 
+# Avoid collecting Python3 only files
+collect_ignore = []
+if env.PY2:
+    collect_ignore.append("test_async.py")
+
 
 def _strip_and_dedent(s):
     """For triple-quote strings"""