Fix coverage version mismatch between sessions

Coverage.py 5.0 fails when faced with a .coverage data file generated by
an earlier version:

  Couldn't use data file 'path/to/.coverage': file is not a database

This affects us because coverage data is generated by the version
specified in poetry.lock (4.5.4), while the session uploading the
coverage data uses the latest version (5.0).

Fix the Nox session by querying Poetry to determine the pinned
coverage.py version, and install this instead.

noxfile.py

@@ -59,7 +59,19 @@ def tests(session: Session) -> None:
 @nox.session(python="3.8")
 def coverage(session: Session) -> None:
     """Upload coverage data."""
-    session.install("coverage", "codecov")
+    output = session.run(
+        "poetry",
+        "export",
+        "--format=requirements.txt",
+        "--without-hashes",
+        "--dev",
+        external=True,
+        silent=True,
+    )
+    (coverage,) = [
+        line for line in output.splitlines() if line.startswith("coverage==")
+    ]
+    session.install(coverage, "codecov")
     session.run("coverage", "xml")
     session.run("codecov", *session.posargs)