Add py3.8 tests

[carver]

What is wrong?

No tests validating 3.8 support.

How can it be fixed

Add new tox environment and circle CI tests.
Also, be sure to add pip constraint blake2b-py v0.1.3:
vyperlang/vyper#1678 (comment)

[veox]

I'm taking a look at this. First step is to get a local virtualenv with Python 3.8.

---

Dependency coincurve has pre-built wheels for Python 3.8 starting with v13 (most recent ATM).

Using an earlier version (like currently-pinned v10) doesn't work on Arch Linux: attempting to build wheel results in error of:

    building '_libsecp256k1' extension
    creating build/temp.linux-x86_64-3.8/build
    creating build/temp.linux-x86_64-3.8/build/temp.linux-x86_64-3.8
    gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -fPIC -I/usr/include/python3.8 -c build/temp.linux-x86_64-3.8/_libsecp256k1.c -o build/temp.linux-x86_64-3.8/build/temp.linux-x86_64-3.8/_libsecp256k1.o
    build/temp.linux-x86_64-3.8/_libsecp256k1.c:493:10: fatal error: secp256k1_ecdh.h: No such file or directory
      493 | #include <secp256k1_ecdh.h>
          |          ^~~~~~~~~~~~~~~~~~
    compilation terminated.
    error: command 'gcc' failed with exit status 1

---

typed-ast wheel build fails:

  gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -fPIC -Iast3/Include -I/usr/include/python3.8 -c ast3/Python/ast.c -o build/temp.linux-x86_64-3.8/ast3/Python/ast.o
  ast3/Python/ast.c:22:1: error: conflicting types for ‘_PyObject_FastCall’
     22 | _PyObject_FastCall(PyObject *func, PyObject *const *args, int nargs)
        | ^~~~~~~~~~~~~~~~~~
  In file included from /usr/include/python3.8/abstract.h:837,
                   from /usr/include/python3.8/Python.h:147,
                   from ast3/Python/ast.c:6:
  /usr/include/python3.8/cpython/abstract.h:145:1: note: previous definition of ‘_PyObject_FastCall’ was here
    145 | _PyObject_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs)
        | ^~~~~~~~~~~~~~~~~~
  error: command 'gcc' failed with exit status 1

This is not a direct dependency; mypy pulls in v1.3.5 (via <1.4.0,>=1.3.1).

Installing mypy-0.740 bypasses the issue. Note, though, this version has had quite some changes compared to currently-pinned v0.701.

---

hypothesis v3 has some internal error with incorrect type arg being passed that I don't feel like getting into. v4 works fine - kind of...

[veox]

Working branch in https://github.com/veox/py-evm/commits/add-python3.8-testing - will re-visit later.

Some stream tests fail due to hypothesis picking a very large int number that doesn't fit into an underlying int. See py38-core job (not started ATM).

---

Actually, the above job doesn't show that, because plyvel doesn't have a pre-built wheel for Python 3.8 yet; and the CI machine doesn't have the .h header file to build the wheel itself.

For same reason, all CI jobs that need database access - py38-{core,database,transactions,vm} - fail.

---

Furthermore, the mypy upgrade has produced quite some new failures in lint jobs; see py36-lint for example.

[veox]

TL;DR: We can finally see core and lint failures - stemming from hypothesis and mypy updates, respectively. These are present for all of py36, py37, py38.

---

I've tried installing libleveldb-dev for Python 3.8 jobs only, using this wonderful way (commit) (via CircleCI YAML docs).

Problem is, it doesn't work for complex value maps: CircleCI only takes the first step. There are topics on CircleCI's forums confirming the fact:

https://discuss.circleci.com/t/yaml-anchors-implementation-incomplete/16916
https://discuss.circleci.com/t/allow-bundling-several-steps-e-g-by-supporting-nested-steps/15339/7

---

So the two remaining options are:

• install libleveldb-dev on all CI jobs, even ones that don't need it;
• wait for plyvel to get Python 3.8 wheels.

I'll now perform a magic trick by doing both at the same time. 😄 🥁

[veox]

Specific to Python 3.8 linting, it turned out pyflakes (a nested dependency) also needs to be updated. However, a tagged version hasn't yet been released.

[veox]

JIC, this has moved to a different PR, #1940.