Exchangelib

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- Make SOAP-based autodiscovery the default, and remove support for POX-based
discovery. This also removes support for autodiscovery on Exchange 2007.
Only `Account(..., autodiscover=True)` is supported again.
- Deprecated `RetryPolicy.may_retry_on_error`. Instead, add custom retry logic
in `RetryPolicy.raise_response_errors`.
- Moved `exchangelib.util.RETRY_WAIT` to `BaseProtocol.RETRY_WAIT`.

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- Added support for SOAP-based autodiscovery, in addition to the existing POX
(plain old XML) implementation. You can specify the autodiscover
implementation explicitly using the `autodiscover` argument:
`Account(..., autodiscover="soap")` or `Account(..., autodiscover="pox")`. POX
is still the default.

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- Added new `OAuth2LegacyCredentials` class to support username/password auth
over OAuth.

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- Fixed token refresh bug with OAuth2 authentication, again

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- Fixed `Protocol.get_free_busy_info()` when called with +100 accounts.
- Allowed configuring DNS timeout for a single nameserver
(`Autodiscovery.DNS_RESOLVER_ATTRS["timeout""]`) and the total query lifetime
(`Autodiscovery.DNS_RESOLVER_LIFETIME`) separately.
- Fixed token refresh bug with OAuth2 authentication

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- Bugfix release