* forms: replace re._pattern_type with re.Pattern
Python 3.7 removed re._pattern_type and replaced it with
re.Pattern.
* readme: update for Python 3.7
* Update requirements
Also remove some unused ones which were neither a direct dependency
nor a dependency of our dependencies.
* account: force ASCII usernames on login form
Our database doesn't like it when we check for unicode data in
a column that stores ASCII data, so let's stop it before it
gets that far.
* Move travis CI to Python 3.7
* travis: use xenial dist
* fix newer linter warnings
Apparently bare excepts are literally Hitler, and we have some
new import sorting rules. Hooray!
* requirements: remove six
This is a dependency for sqlalchemy-utils, but we ourselves don't
depend on it directly because we've never been on Python 2 ever.
* Update requirements.txt
read(1) returns b'' on EOF, which wasn't checked for in a couple cases,
which could cause empty/truncated input to enter one of the `while True`
loops without hope of exit. programming is hard.