tests: check availability of pyflakes by trying to import pyflakes module

Since e397c6d74652, we use the pyflakes module instead of the pyflakes
executable. As was pointed out during the review, the hghave check can be
rewritten to try to import the pyflakes module instead of spawning a new
subprocess.

tests/hghave.py

@@ -574,11 +574,14 b' def has_root():'
 
 @check("pyflakes", "Pyflakes python linter")
 def has_pyflakes():
-    return matchoutput(
-        "sh -c \"echo 'import re' 2>&1 | $PYTHON -m pyflakes\"",
-        br"<stdin>:1: 're' imported but unused",
-        True,
-    )
+    try:
+        import pyflakes
+
+        pyflakes.__version__
+    except ImportError:
+        return False
+    else:
+        return True
 
 
 @check("pylint", "Pylint python linter")