[Feature Request] Basic test units

[tai271828]

Description

I am aware of the empty test folder https://github.com/pycontw/mail_handler/tree/master/tests and this PR also needs regression test when resolving conflicts #4 . Basic test units will absolutely speed up the development of #4 and future PRs.

Possible Solution

pytest in tox.

Related Issue

#4

[tai271828]

I am working on #4 and I will add the corresponding tests at least.