Made eth-abi a primary dependency

[voith]

What was wrong?

eth-tester would give the following error:

In [1]: from eth_tester import EthereumTester
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-1-72c9d4f19b88> in <module>()
----> 1 from eth_tester import EthereumTester

~/Projects/eth-tester/eth_tester/__init__.py in <module>()
      2 import sys
      3
----> 4 from .main import (  # noqa: F401
      5     EthereumTester,
      6 )

~/Projects/eth-tester/eth_tester/main.py in <module>()
     29 )
     30
---> 31 from eth_tester.backends import (
     32     get_chain_backend,
     33 )

~/Projects/eth-tester/eth_tester/backends/__init__.py in <module>()
     10     MockBackend,
     11 )
---> 12 from .pyevm import (  # noqa: F401
     13     PyEVMBackend,
     14     is_pyevm_available,

~/Projects/eth-tester/eth_tester/backends/pyevm/__init__.py in <module>()
      4     is_pyevm_available,
      5 )
----> 6 from .main import (  # noqa: F401
      7     PyEVMBackend,
      8 )

~/Projects/eth-tester/eth_tester/backends/pyevm/main.py in <module>()
     10 import rlp
     11
---> 12 from eth_abi import (
     13     decode_single
     14 )

ModuleNotFoundError: No module named 'eth_abi'

This was missed in #115 (4073ee2).
This wasn't detected by tests because eth-abi was testing dependency

How was it fixed?

eth-abi was removed as a testing dependency and was made a primary dependency.

Bonus changes

• pycryptodome or pysha3 are optional dependencies of py-evm that are needed for eth-tester to work. Install them based on the python implementation available. (Haven't tested it with pypy manually, I'm relying on tests to tell me).
• Added python3.7 to the list of supported python versions.

PS: I have added all these in separate commits to keep unrelated changes isolated.

Cute Animal Picture

[carver]

Just to explicitly state the implicit change: this treats eth-tester like an end product rather than a library. (by forcing a choice of eth-hash backend, for example)

I think this is reasonable, just wanted to make sure everyone is one the same page.

[voith]

@carver My reasoning is as follows:
I checked setup.py in py-evm. The extra-eth requirements in py-evm are:

'eth-extra': [
        "coincurve>=8.0.0,<9.0.0",
        "eth-hash[pysha3];implementation_name=='cpython'",
        "eth-hash[pycryptodome];implementation_name=='pypy'",
        "plyvel==1.0.5",
    ],

Although a user has the freedom to explicitly install any eth-hash backend, installations using eth-extra will bind the backend with the python implementation.
The change I made here is no different.

[voith]

I encountered this in voith/eth-tester-rpc#10

[carver]

That's a good point, the eth-tester[py-evm] extra should probably just be on py-evm[eth-extra] rather than on py-evm and duplicating all those packages here.

[voith]

@carver I had thought about using py-evm[eth-extra]. But there were two extra dependencies, plyvel and coincurve for which there are no tests at the moment.
Also "eth-hash[pysha3];implementation_name=='cpython' and "eth-hash[pycryptodome];implementation_name=='pypy'" are not pinned to any range at the moment in py-evm, which worried me.

Probably, I should have fixed the eth-hash change in py-evm, but to have plylevel as dependency, it would need some tests.