Fixup use with a running event loop (#1420)

* Fixup use with a running event loop

* Python 3.6 support

nbconvert/exporters/webpdf.py

@@ -6,8 +6,11 @@
 import asyncio
 
 from traitlets import Bool
+import concurrent.futures
+
 from .html import HTMLExporter
 
+
 class WebPDFExporter(HTMLExporter):
     """Writer designed to write to PDF files.
 
@@ -36,7 +39,11 @@ def run_pyppeteer(self, html):
         """Run pyppeteer."""
 
         async def main():
-            browser = await self._check_launch_reqs()()
+            browser = await self._check_launch_reqs()(
+                handleSIGINT=False,
+                handleSIGTERM=False,
+                handleSIGHUP=False,
+            )
             page = await browser.newPage()
             await page.waitFor(100)
             await page.goto('data:text/html,'+html, waitUntil='networkidle0')
@@ -66,7 +73,14 @@ async def main():
             await browser.close()
             return pdf_data
 
-        pdf_data = asyncio.get_event_loop().run_until_complete(main())
+        pool = concurrent.futures.ThreadPoolExecutor()
+        # TODO: when dropping Python 3.6, use
+        # pdf_data = pool.submit(asyncio.run, main()).result()
+        def run_coroutine(coro):
+            loop = asyncio.new_event_loop()
+            asyncio.set_event_loop(loop)
+            return loop.run_until_complete(coro)
+        pdf_data = pool.submit(run_coroutine, main()).result()
         return pdf_data
 
     def from_notebook_node(self, nb, resources=None, **kw):