Lmdb

* 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.

* Dowgrade underlying LMDB to 0.9.26. 0.9.27 has a minor defect that will
need to get resolved.

* Migrate CI pipeline from Travis and AppVeyor to Github Actions. Now
includes comprehensive testing across 4 dimensions (OS, Python version,
cpython/CFFI, pure/with mods). Also includes publishing to PyPI.

* Prevent invalid flag combinations when creating a database.

* Add a Cursor.getmulti method with optional buffer support. Contributed by
Will Thompson <willsthompsongmail.com>.

* Upgrade underlying LMDB to 0.9.27.

* Start of new semantic versioning scheme. This would be a minor version
bump from the 0.99 release if it were semantically versioned.

* Allow environment copy to take a passed-in transaction. This is the
first released feature that requires a (very small) patch to the
underlying C library. By default, the patch will be applied unless
this module is built with LMDB_PURE environment variable set.

* Fix lmdb.tool encoding issues.

* Fix -l lmdb invocation issue.

* Minor documentation improvements.

* Update LMDB to version 0.9.24.

* Update for Python 3.9 (current release candidate) support.

* Resolve a bug when using cursor.putmulti and append=True on dupsort DBs.

* Allow _Database.flags method to take no arguments since the one argument
wasn't being used.

* Fix that a duplicate argument to a lmdb method would cause an assert.

* Solaris needs include "python.h" as soon as possible. Fix
contributed by Jesús Cea.

* Fix crash under debug cpython when mdb_cursor_open failed