Migrations change column types

[dantownsend]

Piccolo migrations lacked the ability to change a column type. For example, with this schema:

# Old table
class MyTable(Table):
    data = Timestamp()

# New table
class MyTable(Table):
    data = Timestamptz()

The type of the data column has changed. Piccolo migrations will now detect when a column type has changed, and will instruct Postgres to change it.

The issue was reported here:

#56