Remove pyproject.toml from distribution

SQLAlchemy does not want to opt-in to pep-517 at this time
as this would require a custom build backend interface
which we have not built yet, and the standard is not
widely adopted at this time in any case.  Per
[1] [2], the presence of this file indicates a positive opt-in
to pep-517, so it must be omitted from source distributions.

[1] https://pip.pypa.io/en/stable/reference/pip/#pep-517-and-518-support
[2] https://www.python.org/dev/peps/pep-0517/#id5

Fixes: sqlalchemy#5207
Change-Id: If8d26a9edf942047920d273d8be778df7a018b3e

Author: zzzeek

MANIFEST.in

@@ -11,3 +11,7 @@ recursive-include lib *.c *.txt
 
 include README* AUTHORS LICENSE CHANGES* tox.ini
 prune doc/build/output
+
+# don't include pyproject.toml until we
+# have explicitly built a pep-517 backend
+exclude pyproject.toml

doc/build/changelog/unreleased_13/5207.rst

@@ -0,0 +1,11 @@
+.. change::
+    :tags: installer
+    :tickets: 5207
+
+    Ensured that the "pyproject.toml" file is not included in builds, as the
+    presence of this file is a positive direction to pip that pep-517 should be
+    used. As this mode of operation is not well supported by current tools /
+    distros, we would like to explicitly disallow this mode of operation until
+    we have an actual pep517 installation module available.
+
+