Add SetEnviroment, go over GetEnvironment... (#1071)

GetEnvironment:  add empty list and named list forms.

`mathics/__init__.py`: attempt to limit imports of big packages sympy,
numpy and mpmath

Go over doctests for each of these.

mathics/__init__.py

@@ -5,9 +5,9 @@
 from importlib import import_module
 from typing import Dict
 
-import mpmath
-import numpy
-import sympy
+from mpmath import __version__ as mpmath_version
+from numpy import __version__ as numpy_version
+from sympy import __version__ as sympy_version
 
 from mathics.version import __version__
 
@@ -17,10 +17,10 @@
 # if we can't get version information.
 version_info: Dict[str, str] = {
     "mathics": __version__,
-    "mpmath": mpmath.__version__,
-    "numpy": numpy.__version__,
+    "mpmath": mpmath_version,
+    "numpy": numpy_version,
     "python": platform.python_implementation() + " " + sys.version.split("\n")[0],
-    "sympy": sympy.__version__,
+    "sympy": sympy_version,
 }