Topological ordering with priorities

- adds link for Kahn's algorithm
- replaces compute_topological_order by compute_topological_order_v2
- introduce class HeapEntry

Add missing codegen.rst to doc directory

Add pytools.codegen

This change takes common code generation functionality out of
pytools.py_codegen and makes it a separate module.

A language-independent parent class of PythonCodeGenerator is also
added.

The version is bumped to 2020.4

Fix typos in warnings

June monster maint

See merge request !44

# Conflicts:
#   setup.py

Allow complex identifiers in @memoize_in

See merge request !43

Fix a README typo

mo(n)keypatch :-)

setup.py: Drop 3.4, 3.5 support

`__init__.py` uses PEP 526 variable annotations, which are supported only in 3.6+

Work around numpy/numpy#16564 in make_obj_array

See merge request !42

Fixes to obj_array.py

1) Fix a typo
2) Rename decorator for consistency with others

Obj array revamp

See merge request !41