Change registry mechanism to avoid circular imports

[brunobord]

closes #288

• As it's a refactor, tests are completely unchanged and mark no regression
• Changelog amended with a mention describing your changes.

Rationale

In the ticket #288, it appeared that simply importing any given calendar would import all the library because of the "auto-registration" of calendars into the ISO registry.

You just need to run the following code and inspect the last lines of the output to witness it:

from trace import Trace
tracer = Trace()
# make run a simple import for a single country
tracer.run('from workalendar.europe.italy import Italy')
res = tracer.results()
# print results
res.write_results(coverdir='.cache', summary=True)

I've attached the trace-master.log here, which is an extract of the output when running against current master (v5.0.3).
trace-master.log

Improvements

This current PR has a very different output. As you can see in the trace-288.log which corresponds to the same code executed on this very branch.
trace-288.log

Here's an array showing the main differences:

	master code	branch "288-new-registry"
number of lines	229	139
load Africa modules	yes 👎	no 🆗
load America modules	yes 👎	no 🆗
load Asia modules	yes 👎	no 🆗
load Europe modules	yes 🆗	yes 🆗
load Oceania modules	yes 👎	no 🆗
load Oceania modules	yes 👎	no 🆗
load USA modules	yes 👎	no 🆗

Important Note The API of the registry module hasn't changed, as all tests show no regression. However, it's expected to change, in order to return plain built-in dict objects instead of OrderedDict objects. A deprecation warning has been added to the Changelog and the code.

[brunobord]

pinging @sevdog, since it was their issue at first.

[brunobord]

aha! I'm taking this 👍 as a "yes, that's cool, you can squash and merge!" (too bad github doesn't send a small notification on emoji reactions. Something like a cool sound when you receive one)

[sevdog]

@brunobord Yes, good job! 👍