* Update bundled LMDB to 0.9.29.
* Add non-bundled testing to CI.
* Remove wheel generation for 2.7 because the manylinux images no longer
support it.
* Allow passing None as a value to transaction.del in CFFI implementation
for parity with cpython implementation.
* Fix Cursor.put behavior on a dupsort DB with append=True.
* Add warning to docs about use of Environment.set_mapsize. This is currently
an unresolved issue with upstream LMDB.
* CFFI implementation: fix a seg fault when open_db returns map full.
* CFFI implementation: fix a bug in open_db in a read-only environment.