Use __all__ and __dir__ to hide non-public API.

Closes #324.

Note that non-public symbols are still accessible when referred to
explicitly by name. But here we use the module-level __dir__()
function to define which symbols are seen when using dir(<module-name>).
This is a Python >= 3.7 only feature, but the function is just ignored
on Python 3.6.

demes/__init__.py

@@ -31,3 +31,30 @@
     dump_all,
 )
 from .ms import from_ms
+
+__all__ = [
+    "__version__",
+    "Builder",
+    "Epoch",
+    "AsymmetricMigration",
+    "Pulse",
+    "Deme",
+    "Graph",
+    "Split",
+    "Branch",
+    "Merge",
+    "Admix",
+    "load_asdict",
+    "loads_asdict",
+    "load",
+    "loads",
+    "load_all",
+    "dump",
+    "dumps",
+    "dump_all",
+    "from_ms",
+]
+
+
+def __dir__():
+    return sorted(__all__)

demes/hypothesis_strategies.py

@@ -8,6 +8,12 @@
 
 import demes
 
+__all__ = ["graphs"]
+
+
+def __dir__():
+    return sorted(__all__)
+
 
 def prec32(x):
     """truncate x to the nearest single-precision floating point number"""