[FIX] *: remove old deprecated modules/functions

PEP-594 is deprecating a bunch of modules. As part of the cleanup, we
are also dealing with long deprecated modules, functions and aliases.

* `assert_` -> `assertTrue`
* `assertEquals` -> `assertEqual`
* `assertNotEquals` -> `assertNotEqual`
* `assertAlmostEquals` -> `assertAlmostEqual`
* `assertRaisesRegexp` -> `assertRaisesRegex`
* `assertRegexpMatches` -> `assertRegex`
* `base64.encodestring` -> `base64.encodebytes`
* `base64.decodestring` -> `base64.decodebytes`
* `inspect.getargspec` -> `inspect.signature`
* `inspect.formatargspec` -> `inspect.signature`
* `logging.warn` -> `logging.warning`

closes odoo/odoo#36863

Task: 2003936
Signed-off-by: Raphael Collet (rco) <rco@openerp.com>