Calls to `alembic` still reference legacy version

[david-pettifor-nd]

The current version of alembic has removed the direct migration class call, so new installations of this utility will fail to run unless a legacy version of alembic is specified during the pip installation.

See traceback:

Traceback (most recent call last):
  File "/usr/local/bin/commcare-export", line 8, in <module>
    sys.exit(entry_point())
  File "/usr/local/lib/python3.8/site-packages/commcare_export/cli.py", line 361, in entry_point
    main(sys.argv[1:])
  File "/usr/local/lib/python3.8/site-packages/commcare_export/cli.py", line 143, in main
    exit(main_with_args(args))
  File "/usr/local/lib/python3.8/site-packages/commcare_export/cli.py", line 352, in main_with_args
    exit_status = evaluate_query(env, query)
  File "/usr/local/lib/python3.8/site-packages/commcare_export/cli.py", line 271, in evaluate_query
    lazy_result = query.eval(env)
  File "/usr/local/lib/python3.8/site-packages/commcare_export/minilinq.py", line 144, in eval
    return self.body.eval(env.bind(self.name, self.value.eval(env)))
  File "/usr/local/lib/python3.8/site-packages/commcare_export/minilinq.py", line 434, in eval
    env.emit_table(TableSpec(
  File "/usr/local/lib/python3.8/site-packages/commcare_export/env.py", line 133, in emit_table
    except CannotEmit: return self.right.emit_table(table_spec)
  File "/usr/local/lib/python3.8/site-packages/commcare_export/env.py", line 556, in emit_table
    self.writer.write_table(table_spec)
  File "/usr/local/lib/python3.8/site-packages/commcare_export/writers.py", line 536, in write_table
    self.make_table_compatible(table_name, row_dict, data_type_dict)
  File "/usr/local/lib/python3.8/site-packages/commcare_export/writers.py", line 456, in make_table_compatible
    ctx = alembic.migration.MigrationContext.configure(self.connection)
AttributeError: module 'alembic' has no attribute 'migration'