Address warnings emitted after SQLAlchemy upgrade to 1.4

[grahamalama]

Since upgrading SQLAlchemy to 1.4 (#502), it has started emitting some warnings:

tests/unit/test_acoustic_service.py::test_ctms_to_acoustic_no_product
  /Users/gbeckley/github/mozilla-it/ctms-api/migrations/env.py:68: RemovedIn20Warning: Deprecated API features detected! These feature(s) are not compatible with SQLAlchemy 2.0. To prevent incompatible upgrades prior to updating applications, ensure requirements files are pinned to "sqlalchemy<2.0". Set environment variable SQLALCHEMY_WARN_20=1 to show all deprecation warnings.  Set environment variable SQLALCHEMY_SILENCE_UBER_WARNING=1 to silence this message. (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    with connectable.connect() as connection:

tests/unit/test_acoustic_service.py::test_ctms_to_acoustic_no_product
  /Users/gbeckley/github/mozilla-it/ctms-api/ctms/crud.py:414: SAWarning: relationship 'Email.stripe_customer' will copy column emails.email_id to column fxa.email_id, which conflicts with relationship(s): 'Email.fxa' (copies emails.email_id to fxa.email_id). If this is not the intention, consider if these relationships should be linked with back_populates, or if viewonly=True should be applied to one or more if they are read-only. For the less common case that foreign key constraints are partially overlapping, the orm.foreign() annotation can be used to isolate the columns that should be written towards.   To silence this warning, add the parameter 'overlaps="fxa"' to the 'Email.stripe_customer' relationship. (Background on this error at: https://sqlalche.me/e/14/qzyx)
    db_email = Email(**email.dict())

tests/unit/test_acoustic_service.py::test_ctms_to_acoustic_no_product
  /Users/gbeckley/github/mozilla-it/ctms-api/ctms/crud.py:414: SAWarning: relationship 'FirefoxAccount.email' will copy column emails.email_id to column fxa.email_id, which conflicts with relationship(s): 'Email.stripe_customer' (copies emails.email_id to fxa.email_id). If this is not the intention, consider if these relationships should be linked with back_populates, or if viewonly=True should be applied to one or more if they are read-only. For the less common case that foreign key constraints are partially overlapping, the orm.foreign() annotation can be used to isolate the columns that should be written towards.   To silence this warning, add the parameter 'overlaps="stripe_customer"' to the 'FirefoxAccount.email' relationship. (Background on this error at: https://sqlalche.me/e/14/qzyx)
    db_email = Email(**email.dict())

tests/unit/test_acoustic_service.py::test_ctms_to_acoustic_no_product
  /Users/gbeckley/github/mozilla-it/ctms-api/ctms/crud.py:414: SAWarning: relationship 'StripeCustomer.email' will copy column emails.email_id to column fxa.email_id, which conflicts with relationship(s): 'Email.fxa' (copies emails.email_id to fxa.email_id), 'FirefoxAccount.email' (copies emails.email_id to fxa.email_id). If this is not the intention, consider if these relationships should be linked with back_populates, or if viewonly=True should be applied to one or more if they are read-only. For the less common case that foreign key constraints are partially overlapping, the orm.foreign() annotation can be used to isolate the columns that should be written towards.   To silence this warning, add the parameter 'overlaps="email,fxa"' to the 'StripeCustomer.email' relationship. (Background on this error at: https://sqlalche.me/e/14/qzyx)
    db_email = Email(**email.dict())

We should fix the issues as outlined in these warnings.