tox: no need to specify basepython manually - remove.

`basepython` is already specified in `testenv` (the first
listed one in the file), and gets set for environments based
on a factor present in the name.

E.g., `testenv:py37-trinity-integration` has 3 factors:

* py37
* trinity
* integration

See `tox` ref:

https://tox.readthedocs.io/en/latest/config.html#factors-and-factor-conditional-settings

Author: veox

tox.ini

@@ -72,20 +72,17 @@ commands=
     py.test -n 1 {posargs:tests/trinity/integration/}
 
 [testenv:py36-trinity-integration]
-basepython=python3.6
 deps = {[common-trinity-integration]deps}
 passenv = {[common-trinity-integration]passenv}
 commands = {[common-trinity-integration]commands}
 
 [testenv:py37-trinity-integration]
-basepython=python3.7
 deps = {[common-trinity-integration]deps}
 passenv = {[common-trinity-integration]passenv}
 commands = {[common-trinity-integration]commands}
 
 
 [testenv:py36-benchmark]
-basepython=python3.6
 deps = .[eth-extras, benchmark]
 commands=
     benchmark: {toxinidir}/scripts/benchmark/run.py
@@ -98,7 +95,6 @@ commands=
     flake8 {toxinidir}/eth
 
 [testenv:lint-py35]
-basepython=python3.5
 deps = {[common-lint]deps}
 setenv = {[common-lint]setenv}
 commands=
@@ -108,7 +104,6 @@ commands=
     mypy --follow-imports=silent --ignore-missing-imports --no-strict-optional --check-untyped-defs --disallow-incomplete-defs -p eth
 
 [testenv:lint-py36]
-basepython=python3.6
 deps = {[common-lint]deps}
 setenv = {[common-lint]setenv}
 commands=