Call towncrier via python instead of doing poor path reconstruction (#1848)

Trying to append an executable suffix to towncrier does not work
on Linux, as it can create absurd names like `towncrier.7` (`.7` coming
from `python3.7`).  Let's instead call it via `python -m towncrier`.
This should avoid relying on correct `PATH` (as I presume the original
code is meant to), while working correctly on all systems
and not requiring Python and towncrier to reside in the same directory.

Fixes #1847

Signed-off-by: Bernat Gabor <[email protected]>

Author: mgorny

docs/conf.py

@@ -59,9 +59,9 @@
 
 def generate_draft_news():
     root = Path(__file__).parents[1]
-    exe = Path(sys.executable)
-    towncrier = exe.with_name("towncrier{}".format(exe.suffix))
-    new = subprocess.check_output([str(towncrier), "--draft", "--version", "NEXT"], cwd=root, universal_newlines=True)
+    new = subprocess.check_output(
+        [sys.executable, "-m", "towncrier", "--draft", "--version", "NEXT"], cwd=root, universal_newlines=True,
+    )
     (root / "docs" / "_draft.rst").write_text("" if "No significant changes" in new else new)