fix: add support for "with .. as .." statement for cleaner exception …

[rhajek]

Closes #190

This PR adds __enter__ and __exit__ methods to support with .. as ..: statement.

        with InfluxDBClient.from_env_properties(self.debug) as client:
            with client.write_api(write_options=WriteOptions(write_type=WriteType.batching)) as write_api:
              write_api.write(bucket="my-bucket",
                                record=Point("h2o_feet")
                                .tag("location", "coyote_creek")
                                .field("level water_level", 5.0))

[codecov]

Codecov Report

Merging #218 (591bd60) into master (d465dda) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   90.75%   90.99%   +0.23%     
==========================================
  Files          26       26              
  Lines        2012     2020       +8     
==========================================
+ Hits         1826     1838      +12     
+ Misses        186      182       -4     

Impacted Files	Coverage Δ
influxdb_client/client/influxdb_client.py	99.29% <100.00%> (+0.02%)	⬆️
influxdb_client/client/write_api.py	99.05% <100.00%> (+1.46%)	⬆️
influxdb_client/client/flux_table.py	88.88% <0.00%> (+2.22%)	⬆️

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[bednar]

Could you please also update README.rst and sources in /example dir?