Use of hypothesis type annotations forces downstream projects to install hypothesis

[leifwar]

Describe the bug
A clear and concise description of what the bug is.

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera.
• (optional) I have confirmed this bug exists on the master branch of pandera.

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Code Sample, a copy-pastable example

In 1fc05de a type annotation SearchStrategy[pd.Series] was added to series_strategy
(pandera/strategies/pandas_strategies.py). This forces downstream projects to install
hypothesis as the place holder for SearchStrategy defined in the same file does not
contain definition of __getitem__.

import pandera as pa

Importing pandera in a downstream project where hypothesis is not available will give the following error

../pandera/pandera/__init__.py:5: in <module>
    from pandera.accessors import pandas_accessor
../pandera/pandera/accessors/pandas_accessor.py:7: in <module>
    from pandera.api.pandas.array import SeriesSchema
../pandera/pandera/api/pandas/array.py:10: in <module>
    from pandera import strategies as st
../pandera/pandera/strategies/__init__.py:3: in <module>
    from pandera.strategies.pandas_strategies import *
../pandera/pandera/strategies/pandas_strategies.py:830: in <module>
    ) -> SearchStrategy[pd.Series]:
E   TypeError: type 'SearchStrategy' is not subscriptable

If hypothesis is available this does not happen.

Expected behavior

It should be possible to use pandera without installing hypothesis.