Move to Python 3.7 and update dependencies (#551)

* 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

nyaa/forms.py

@@ -11,8 +11,9 @@ from wtforms import (BooleanField, HiddenField, PasswordField, SelectField, Stri
                      SubmitField, TextAreaField)
 from wtforms.validators import (DataRequired, Email, EqualTo, Length, Optional, Regexp,
                                 StopValidation, ValidationError)
+from wtforms.widgets import HTMLString  # For DisabledSelectField
 from wtforms.widgets import Select as SelectWidget  # For DisabledSelectField
-from wtforms.widgets import HTMLString, html_params  # For DisabledSelectField
+from wtforms.widgets import html_params
 
 import dns.exception
 import dns.resolver
@@ -78,7 +79,7 @@ def register_email_blacklist_validator(form, field):
     validation_exception = StopValidation('Blacklisted email provider')
 
     for item in email_blacklist:
-        if isinstance(item, re._pattern_type):
+        if isinstance(item, re.Pattern):
             if item.search(email):
                 raise validation_exception
         elif isinstance(item, str):