Specify a minimum tox version

[jranieri-grammatech]

Versions of tox before 3.18.1 are incompatible due to a bug in tox.

Here is what the error looks like:

.package create: /data/jranieri/chisel_work/exceptiongroup/.tox/.package
.package installdeps: flit_scm
ERROR: invocation failed (exit code 1), logfile: /data/jranieri/chisel_work/exceptiongroup/.tox/.package/log/.package-2.log
================================================================================================== log start ===================================================================================================
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/tox/helper/build_requires.py", line 7, in <module>
    backend = __import__(backend_spec, fromlist=[None])
  File "<frozen importlib._bootstrap>", line 1033, in _handle_fromlist
TypeError: Item in ``from list'' must be str, not NoneType

=================================================================================================== log end ====================================================================================================
ERROR: FAIL could not package project - v = InvocationError('/data/jranieri/chisel_work/exceptiongroup/.tox/.package/bin/python /usr/lib/python3/dist-packages/tox/helper/build_requires.py flit_scm buildapi', 1)

Specifying a minimum version of tox will make older versions of tox create a provisioning environment that contains a newer, compatible, version of tox -- making everything transparently "just work".

[coveralls]

Pull Request Test Coverage Report for Build 3491844464

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.463%

---

Totals
Change from base Build 3470321442:	0.0%
Covered Lines:	2049
Relevant Lines:	3130

---

💛 - Coveralls

[agronholm]

Thanks!